This paper presents an improved technique for determining complex permittivity using the transmission/reflection (TR) method. The TR method is a widely used broadband measurement technique for determining complex permittivity and permeability. The paper examines the special case of permittivity measurement and presents new robust algorithms that eliminate the instability of commonly used procedures at frequencies corresponding to integer multiples of one-half wavelength in the sample. An error analysis is provided to estimate errors due to uncertainties in scattering parameters, length measurement, and reference plane position. New equations are derived for determining complex permittivity independent of reference plane position and sample length. The paper also addresses the problem of transforming S-parameter measurements from calibration reference planes to the ends of the sample. This transformation requires knowledge of the sample's position in the sample holder, which may be limited in many applications. Equations that are independent of reference plane position and sample length are desirable for high-temperature applications. The goal of this paper is to examine the scattering equations in detail and present an improved method for solving the transmission line equations in an iterative fashion with application to permittivity measurements. Second, to derive scattering equations that are simultaneously invariant to the position of the reference planes and sample length. Third, to present an uncertainty analysis for this new procedure. The paper presents a new procedure for obtaining complex permittivity from the scattering equations that is stable over the frequency spectrum. This procedure minimizes the instability of the Nicolson–Ross–Weir equations by setting μ_R* = 1. This new procedure allows measurements to be taken on samples of arbitrary length. The paper also presents equations that are independent of reference plane position and sample length. These equations are useful for high-temperature applications. The paper also presents an uncertainty analysis for the new procedure, which includes error sources such as measurement errors in the magnitude and phase of the scattering parameters, line losses, connector mismatch, and uncertainty in reference plane positions. The paper concludes that the new procedure is robust for both high-loss and low-loss materials and that the uncertainty in the permittivity measurement decreases as the sample length increases. The paper also discusses the importance of using equations that are invariant to reference plane position and sample length for high-temperature applications.This paper presents an improved technique for determining complex permittivity using the transmission/reflection (TR) method. The TR method is a widely used broadband measurement technique for determining complex permittivity and permeability. The paper examines the special case of permittivity measurement and presents new robust algorithms that eliminate the instability of commonly used procedures at frequencies corresponding to integer multiples of one-half wavelength in the sample. An error analysis is provided to estimate errors due to uncertainties in scattering parameters, length measurement, and reference plane position. New equations are derived for determining complex permittivity independent of reference plane position and sample length. The paper also addresses the problem of transforming S-parameter measurements from calibration reference planes to the ends of the sample. This transformation requires knowledge of the sample's position in the sample holder, which may be limited in many applications. Equations that are independent of reference plane position and sample length are desirable for high-temperature applications. The goal of this paper is to examine the scattering equations in detail and present an improved method for solving the transmission line equations in an iterative fashion with application to permittivity measurements. Second, to derive scattering equations that are simultaneously invariant to the position of the reference planes and sample length. Third, to present an uncertainty analysis for this new procedure. The paper presents a new procedure for obtaining complex permittivity from the scattering equations that is stable over the frequency spectrum. This procedure minimizes the instability of the Nicolson–Ross–Weir equations by setting μ_R* = 1. This new procedure allows measurements to be taken on samples of arbitrary length. The paper also presents equations that are independent of reference plane position and sample length. These equations are useful for high-temperature applications. The paper also presents an uncertainty analysis for the new procedure, which includes error sources such as measurement errors in the magnitude and phase of the scattering parameters, line losses, connector mismatch, and uncertainty in reference plane positions. The paper concludes that the new procedure is robust for both high-loss and low-loss materials and that the uncertainty in the permittivity measurement decreases as the sample length increases. The paper also discusses the importance of using equations that are invariant to reference plane position and sample length for high-temperature applications.