We have measured the temperature of laser-cooled Rb atoms in optical molasses as a function of laser intensity and detuning. For both ⁸⁵Rb and ⁸⁷Rb, cooled on the F = 3 → F' = 4 and F = 2 → F' = 3 transitions, respectively, the temperatures are proportional to the ratio of laser power and detuning for a wide range of these parameters. We observe a small but significant difference between the two isotopes. We also show the results of three-dimensional semi-classical numerical calculations. Our results favor a model which includes atomic localization in optical standing waves.