The potential of an in situ photopolymerized hexylacrylate-based monolithic stationary phase-bearing sulfonic acid groups was investigated by studying the chromatographic retention of small structurally related peptides (enkephalins) by nano-LC. Several retention mechanisms were highlighted. First, a reverse-phase chromatographic behavior toward neutral solutes due to hexylacrylate-moieties was demonstrated. Second, an evaluation of the influences of buffer pH suggested the involvement of a cation-exchange mechanism due to the presence of 2-acrylamido-2-methyl-1-propanesulfonic acid. This cation-exchange phenomenon was confirmed by the clear influence of Na(+) concentration in the mobile phase on peptide retention.