Effective masses derived from the low-field Shubnikov-de Haas oscillations in high mobility GaAs/AlGaAs samples are found to decrease systematically with density. Measurements in a low-density sample show the temperature dependence does not conform to standard theory. It is argued that this is to be expected when there is extra structure, in addition to the Landau levels, in the density of states. This may be important in interpreting measurements of composite fermion effective masses.