Stable isotopes are increasingly used as tracers in environmental studies to determine the proportional contribution of multiple sources to a mixture. When the number of potential sources exceeds n+1, a unique solution cannot be determined. To address this, a method is proposed that identifies feasible source contributions by examining all possible combinations of source proportions (0–100%) in small increments. Combinations that match the observed isotopic signature within a small tolerance are considered feasible solutions. The frequency and range of these solutions are then determined. Users should report the distribution of feasible solutions rather than a single value like the mean. This method was applied to various environmental studies, including plant water use, geochemistry, air pollution, and dietary analysis. It provides the range of isotopically determined source contributions, which can be further restricted by additional non-isotopic constraints. The range depends on the geometry of the mixing space and the similarity of source and mixture isotopic signatures. A sensitivity analysis showed that the estimated ranges vary only modestly with different choices of source increment and mass balance tolerance. A computer program, IsoSource, is available to perform these calculations. The method allows for the determination of all possible source contributions when the number of sources exceeds n+1, and it has been demonstrated in various ecological applications. The procedure involves iteratively creating combinations of source proportions, calculating predicted isotopic signatures, comparing them to observed values, and describing the distribution of feasible solutions. The method is robust to outliers and provides a more accurate representation of source contributions.Stable isotopes are increasingly used as tracers in environmental studies to determine the proportional contribution of multiple sources to a mixture. When the number of potential sources exceeds n+1, a unique solution cannot be determined. To address this, a method is proposed that identifies feasible source contributions by examining all possible combinations of source proportions (0–100%) in small increments. Combinations that match the observed isotopic signature within a small tolerance are considered feasible solutions. The frequency and range of these solutions are then determined. Users should report the distribution of feasible solutions rather than a single value like the mean. This method was applied to various environmental studies, including plant water use, geochemistry, air pollution, and dietary analysis. It provides the range of isotopically determined source contributions, which can be further restricted by additional non-isotopic constraints. The range depends on the geometry of the mixing space and the similarity of source and mixture isotopic signatures. A sensitivity analysis showed that the estimated ranges vary only modestly with different choices of source increment and mass balance tolerance. A computer program, IsoSource, is available to perform these calculations. The method allows for the determination of all possible source contributions when the number of sources exceeds n+1, and it has been demonstrated in various ecological applications. The procedure involves iteratively creating combinations of source proportions, calculating predicted isotopic signatures, comparing them to observed values, and describing the distribution of feasible solutions. The method is robust to outliers and provides a more accurate representation of source contributions.