Fractional quantum-Hall-effect features around filling factor ν=½ have been analyzed using the composite-fermion approach. Effective masses deduced from the temperature dependence of the Shubnikov–de Haas (SdH) oscillations, in agreement with other measurements, show a divergence as the filling factor approaches ν=½ and scale as (density)¹/². The magnetic-field dependence of the amplitude is explained quantitatively in terms of normal impurity scattering and a strong dephasing term associated with density inhomogeneities of order 0.5%. It is pointed out that assumptions made in the derivation of the standard theory used to analyze SdH oscillations are less likely to be satisfied for composite fermions and that some caution should therefore be used in interpreting effective-mass results obtained in this way.