To develop an accurate and rapid evaluation of distillation yield curves of heavy crude oils, simulated distillation by gas chromatography (SIMDlS GC) curves were performed for the first time in Mexican Istmo and Maya crude oil samples. In this study, we found large uncertainties using ASTM D-5307, and some improvements to the technique were applied as alternatives to measure the assay distillation. These results were compared with the curves obtained from the physical distillation method outlined in ASTM Method D-2892, showing good agreement. The statistical analysis of the SIMDIS data is addressed as a good alternative for measuring these heavy crude oils.