Conventional methods of assessing organic matter (OM) thermal maturity have limitations and often fail to reflect the geochemical heterogeneity between individual organic phases in mudstone samples. Vitrinite and solid bitumen reflectance, for example, is often measured to estimate OM thermal maturity but is restricted to specific types of OM. Alternative methods deserve exploration. This dataset provides the optical reflectance values that support a study investigating the application of fine-scale geochemical techniques to estimate OM thermal maturity. The study proposes a new analytical procedure using field emission-electron probe micro-analyzer (FE-EPMA). This instrument analyzes both the major elemental composition of the individual OM grains and the carbon X-ray spectrum. Past works suggest that the carbon X-ray spectrum may be sensitive to thermal maturity or OM types. To test this method, we analyzed a suite of mudstone samples with independent measurements of thermal maturity (i.e., vitrinite and solid bitumen reflectance) and compared a number of different spectral features. This data release provides the known vitrinite and solid bitumen optical reflectance values used in the analysis. The first data table contains the mean and standard deviation of multiple measurements taken from various samples reported in prior works (Birdwell and Wilson 2019, French et al. 2020, Zhou et al. 2022). The second data table is a collection of individual reflectance values, the maceral group and OM type from targeted spots for sample: Calumet Collier 34-5 13574 (Hackley et al. 2024). For the full study, refer to the larger work, "Using X-ray spectrum of carbon in electron micro-probe analysis to determine thermal maturity of organic matter in mudstones".