Temperature (T in °C) and mean vitrinite reflectance (Rm in %) of sedimentary organic matter samples from six liquid-dominated geothermal systems plot about a line approximated by a regression equation of the form Rm = 0.435 exp 0.00683T. The reflectance data from these systems are strongly temperature-dependent, with a coefficient of determination (r2) of 0.8. Geologic evidence indicates that reaction duration ranges from 103 or 104 to 106 yr in these systems that appear to have near-maximum temperatures. The uncertainty in vitrinite reflectance and temperature determinations must account for a large part of the remaining variability not explained by the regression equation; therefore, the strong temperature dependence of Rm indicates that after about 104 yr, reaction duration has little or no influence on metamorphism of organic matter in liquid-dominated geothermal systems. These data indicate that vitrinite reflectance can be used to determine the maximum temperature reached in hot sedimentary basins of moderate longevity.