Vitrinite reflectance (VR_o; %) generally is considered the most reliable technique to determine the thermal maturity of sedimentary rocks. However, it is a time-consuming process to collect reflectance (R_o; %) measurements and is subjective to the interpretation of each trained technician, who must be able to discern between vitrinite and solid bitumen and other organic matter types. Inadvertent misidentification of solid bitumen for vitrinite can lead to reports of ‘suppressed’ VR_o, especially at lower thermal maturities (< 1.0% R_o). Programmed pyrolysis data, such as T_max and hydrogen index (HI), are comparatively inexpensive and more time-efficient to obtain than R_o data and are determined by instrument settings, rather than by operator decision, and are therefore independent of operator-based training or experience bias. This study uses hydrous pyrolysis (HP) residues from various coals and shales to relate measured VR_o and solid bitumen reflectance (BR_o; %) values to their respective T_max and HI values and determines whether these relationships can be used as a proxy to calculate R_o in naturally matured samples. Although the estimation of R_o is not always accurate, the results demonstrate that relational equations for shales and coals derived from the T_max and HI data of HP residues can effectively calculate R_o in natural series. Approximately 60% of calculated R_o from T_max and 83% of calculated R_o from HI relational equations are within interlaboratory reproducibility limits (± 0.2% shale BR_o; ± 0.06% coal VR_o) when compared to their respective measured R_o values from natural series. Variables that may affect accuracy of the applied relational equations include variable sedimentary organic matter composition of samples, differences of maturation reaction kinetics of the sedimentary organic matter in experimental versus natural settings, and decreasing reliability of all thermal proxy measurements at higher maturities.