The paper presents the refined analysis of the cosmic microwave background (CMB) spectrum using data from the FIRAS instrument aboard the COBE satellite. The FIRAS measures the difference between the CMB and a blackbody spectrum, providing new tighter upper limits on deviations from a blackbody. The results show that the RMS deviations are less than 50 parts per million of the peak of the CMBR. The Comptonization parameter y is found to be less than 15×10⁻⁶ and the chemical potential μ is less than 9×10⁻⁵ at 95% confidence level. The absolute temperature of the CMBR is determined to be 2.728 ± 0.004 K, and the dipole direction is (ℓ, b) = (264.14° ± 0.30, 48.26° ± 0.30). The dipole amplitude is 3.372 ± 0.007 mK. These results agree with previous publications. The FIRAS instrument is a polarizing Michelson interferometer that measures the spectral difference between a 7° patch of sky and an internal blackbody. The calibration process was improved, and the data were processed through sorting, deglitching, averaging, calibration, and destripping. The data were analyzed to determine the CMBR temperature and dipole spectrum. The CMBR temperature was determined to be 2.728 ± 0.004 K, and the dipole amplitude was found to be 3.369 ± 0.004 mK. The dipole direction was consistent with previous results. The analysis of the sky data showed that the CMBR spectrum is well fit by a Planck blackbody spectrum, with small deviations consistent with earlier results. The results also show that the CMBR spectrum is consistent with a thermal origin and a Doppler shift. The dipole temperature, 2.717 ± 0.007 K, agrees with the monopole temperature. The Doppler shift implies that the Sun's peculiar velocity relative to the comoving frame is 371 ± 1 km/s toward (ℓ, b) = (264.14° ± 0.15, 48.26° ± 0.15). The CMBR temperature is 2.728 ± 0.004 K, with the error dominated by thermometry uncertainties. The weighted RMS deviation from the fit is 50 ppm of the peak brightness. The limits on |y| and |μ| are 15×10⁻⁶ and 9×10⁻⁵, respectively, at 95% confidence level. These results place stringent constraints on theoriesThe paper presents the refined analysis of the cosmic microwave background (CMB) spectrum using data from the FIRAS instrument aboard the COBE satellite. The FIRAS measures the difference between the CMB and a blackbody spectrum, providing new tighter upper limits on deviations from a blackbody. The results show that the RMS deviations are less than 50 parts per million of the peak of the CMBR. The Comptonization parameter y is found to be less than 15×10⁻⁶ and the chemical potential μ is less than 9×10⁻⁵ at 95% confidence level. The absolute temperature of the CMBR is determined to be 2.728 ± 0.004 K, and the dipole direction is (ℓ, b) = (264.14° ± 0.30, 48.26° ± 0.30). The dipole amplitude is 3.372 ± 0.007 mK. These results agree with previous publications. The FIRAS instrument is a polarizing Michelson interferometer that measures the spectral difference between a 7° patch of sky and an internal blackbody. The calibration process was improved, and the data were processed through sorting, deglitching, averaging, calibration, and destripping. The data were analyzed to determine the CMBR temperature and dipole spectrum. The CMBR temperature was determined to be 2.728 ± 0.004 K, and the dipole amplitude was found to be 3.369 ± 0.004 mK. The dipole direction was consistent with previous results. The analysis of the sky data showed that the CMBR spectrum is well fit by a Planck blackbody spectrum, with small deviations consistent with earlier results. The results also show that the CMBR spectrum is consistent with a thermal origin and a Doppler shift. The dipole temperature, 2.717 ± 0.007 K, agrees with the monopole temperature. The Doppler shift implies that the Sun's peculiar velocity relative to the comoving frame is 371 ± 1 km/s toward (ℓ, b) = (264.14° ± 0.15, 48.26° ± 0.15). The CMBR temperature is 2.728 ± 0.004 K, with the error dominated by thermometry uncertainties. The weighted RMS deviation from the fit is 50 ppm of the peak brightness. The limits on |y| and |μ| are 15×10⁻⁶ and 9×10⁻⁵, respectively, at 95% confidence level. These results place stringent constraints on theories