For sample mounting, organic petrology laboratories typically use cold-setting epoxy-resin (e.g., 40 °C, used by Oklahoma Geological Survey, OGS) or heat-setting thermoplastic (e.g., 180 °C, used by U.S. Geological Survey, USGS). Previous workers have suggested a systematic huminite/vitrinite reflectance (VR_o) increase was associated with the thermoplastic preparation process, relative to epoxy mounting, which was possibly attributed to moisture loss from organic matter due to the transient high temperatures of plastic mounting. In this study, we evaluated thermal effects to low thermal maturity organic matter from transient exposure to elevated temperatures. A subbituminous coal sample was subjected to long-term (4 to 38 weeks) exposure to temperatures of 85 to 120 °C and afterward evaluated by multiple approaches to test thermal advance [elemental analyses, Rock-Eval pyrolysis, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), pyrolysis gas chromatography, and petrographic analyses, including huminite/vitrinite reflectance and spectral fluorescence], all of which showed no detectable systematic (statistically insignificant) changes between the original sample and its heat-treated products. We also compared huminite/vitrinite reflectance of six low thermal maturity samples (those most likely to react to transient heating) mounted via both cold-setting epoxy-resin and heat-setting thermoplastic. Results indicate measured VR_o of a sample prepared by one mounting process was within the standard deviation of reflectance for the same sample prepared via the other process. Moreover, VR_o results were not systematically higher in thermoplastic mounts. Contrary to previous work, these results suggest thermoplastic mounting or other transient exposure to elevated temperatures does not impact thermal maturity estimates from reflectance measurement for low thermal maturity organic samples. Furthermore, the average interlaboratory difference in measured VR_o (between OGS and USGS) for the same sample prepared by either epoxy-resin or thermoplastic mounting was 0.038%, about double the average difference between VR_o for the same sample prepared via epoxy-resin versus thermoplastic in a single laboratory (0.024%). This result indicates interlaboratory variability impacts VR_o measurement reproducibility to the extent that systematic differences could not be observed between thermoplastic and cold-setting sample preparation approaches, even if such differences were present.