The effective mass of electrons in (100) Si inversion and accumulation layers was measured by the temperature broadening of oscillatory magnetoconductance technique for a wide range of substrate doping level, geometry, and interface charge as well as various field-strength and substrate biases. It is shown that the apparent effective mass values are very sensitive to these parameters. Measurements were made at constant magnetic field but for varying oxide charge so that for a given magnetic field the effect of oxide-charge scattering could be eliminated by extrapolating to zero oxide charge. The result was remarkably a carrier-concentration independent mass of about 0.21. Collision broadening of the oscillations was studied to determine the scattering time τ. The values of τ were found to be sensitive to the preexponential damping factor in H. The results are in fair agreement with the zero-field conductivity and average magnetoconductance measurements. © 1977 The American Physical Society.