We investigated the effect of the background electrolyte anions on the electrophoretic 
mobilities of the cationic amino acids arginine and lysine and the polycationic peptides 
tetraarginine, tetralysine, nonaarginine, and nonalysine. Background electrolytes composed 
of sodium chloride, sodium propane-1,3-disulfonate, and sodium sulfate were used. For the 
amino acids, determination of the limiting mobility by extrapolation, using the Onsager-Fuoss 
theory expression, yielded consistent estimates. For the peptides, however, the estimates of 
the limiting mobilities were found to spuriously depend on the background electrolyte salt. 
This paradox was resolved using molecular modeling. Simulations, on all-atom as well as coarse 
grained levels, show that significant counterion condensation, an effect not accounted for in 
Onsager-Fuoss theory, occurs for the tetra- and nonapeptides, even for low background electrolyte 
concentrations. Including this effect in the quantitative estimation of the background electrolyte 
effect on mobility removed the discrepancy between the estimated limiting mobilities in different 
salts. The counterion condensation was found to be mainly due to electrostatic interactions, with 
specific ion effects playing a secondary role. Therefore, the conclusions are likely to be 
generalizable to other analytes with a similar density of charged groups and Onsager-Fuoss theory 
is expected to fail in a predicable way for such analytes.