Capillary zone electrophoresis was performed on fourteen synthetic opioid peptides and one tachykinin peptide (substance P = SP) at pH values of 2.5, 5.5 and 8.5 to rationalize the electrophoretic behavior of these neuropeptides. A plot of the theoretical q/M(r)2/3 values (where q = peptide charge) calculated across the pH range of 1 to 10 for these peptides was used to understand and to predict their separation. The experimentally determined electrophoretic mobilities (mu) were correlated to the estimate of the relative mu predicted by q/M(r)2/3 and by [ln (q + 1)]/n0.43 (where n = number of constituent amino acids), where q values were calculated using amino acid pKa values for an isolated amino acid and for an amino acid in a peptide. In general, relatively high correlations were obtained with either mathematical expression and with both sets of amino acid pKa values for data at a single pH value. However, the combination of the former expression and the adjusted pKa values gave the best prediction of electrophoretic behavior when data for the three pH values were correlated across these different separation conditions.