Results of the first systematic investigation of differences among reference-quality ellipsometrically measured pseudodielectric function (E) spectra of crystalline Si are discussed. These data are nominally used to approximate the bulk dielectric function of this material for optical modeling. In addition to the expected influence of residual overlayers, we identify surface-local-field and energy-derivative effects even between spectra obtained for seemingly identical H-terminated surfaces. Model calculations indicate that these effects account for nearly all differences among spectra studied. The isotropic contribution to the surface-local-field effect is also reported.