Capillary sodium dodecyl sulfate (SDS)-gel electrophoresis, for fast and high-resolution separations of protein molecules based on their molecular masses was investigated, employing a polymeric sieving network of polyethylene oxide (PEO). A standard mixture of five proteins ranging from 14.2-66.0 kDa in molecular mass was used to evaluate the sieving matrix. A general migration velocity equation was derived for capillary SDS-gel electrophoresis of proteins and supported by the experimental data. This equation gives a better understanding of SDS-protein separations via capillary gel electrophoresis. Results are presented regarding the effects of different operational variables such as gel concentration, electric field strength, molecular mass, and temperature on the electrophoretic migration properties of the different size protein molecules.