Using a test mixture consisting of standard proteins (cytochrome c, chymotrypsinogen A, hen egg albumin, bovine serum albumin, aldolase, catalase and ferritin) and synthetic polypeptides (polylysine, polyaspartic, polyglutamic acid and polyproline) it was revealed that using sodium dodecyl sulfate (SIDS) as background electrolyte modifier at acid pH (2.5) allows selective separation of highly positively charged polypeptides (polylysine) provided that their relative molecular mass is sufficiently low (3300 Da). The altered elution sequence of standard proteins as compared to a separation done without SIDS may help their identification, Addition of Pluronic F127 offers clear-cut separations of standard proteins up to a relative molecular mass of 5 x 101 Da and allows to reveal protein/polypeptide microheterogeneity where applicable. None of the systems tested is suitable for the separation of acidic polypeptides and polyproline.