In wells where the drilled sequence is now at its maximum temperature, relations between depth and vitrinite reflectance show three segments: an upper segment with a linear gradient from 0.2-0.25% Ro at the surface to 0.6-0.7% Ro; a middle segment in which reflectance increases rapidly to c.1.0% R; and a lower segment in which the gradient is again linear but reflectance increases more rapidly than in the upper segment. With a linear scale for depth, the inflection represented by the short middle segment tends to be obscured by the adoption by some authors of a log scale for reflectance. The depth to the inflection systematically increases with decrease in geothermal gradient, allowing the development of a general diagram relating depth, reflectance and geothermal gradient. In wells where erosion is probable either at the present-day surface or at an unconformity, the general diagram can be used to estimate former maximum depths of burial and paleogeothermal gradients. These estimates, together with the presence of the inflection in the depth/reflectance relation, should be part of the input into modelling of the thermal history of sedimentary basins when reflectance is used in the model. The inflection is the result of the changing chemistry of vitrinite during oil generation, and the contrast between the depth/reflectance gradients above and below the inflection comparably reflects the contrast in vitrinite chemistry.