We have measured the apparent mass of (100) Si surface electrons by thermal broadening of SdH oscillations of inversion and accumulation layers for a wide range of substrate doping level, geometry, interface charge density, magnetic field strength and substrate bias. It is shown that the apparent mass values determined in this way are very sensitive to these parameters and others which are not yet understood. Collision broadening of the SdH oscillations has been studied in terms of scattering time τ and compared with zero field mobility as well as average magnetoconductance relaxation times. Fair agreements were obtained between these quantities. © 1978.