The influence of the position of the amino acid proline in polypeptide sequences is examined by a combination of ion mobility spectrometry-mass spectrometry (IMS-MS), amino acid substitutions, and molecular modeling. The results suggest that when proline exists as the second residue from the N-terminus (i.e., penultimate proline), two families of conformers are formed. We demonstrate the existence of these families by a study of a series of truncated and mutated peptides derived from the 11-residue peptide Ser(1)-Pro(2)-Glu(3)-Leu(4)-Pro(5)-Ser(6)-Pro(7)-Gln(8)-Ala(9)-Glu(10)-Lys(11). We find that every peptide from this sequence with a penultimate proline residue has multiple conformations. Substitution of Ala for Pro residues indicates that multiple conformers arise from the cis-trans isomerization of Xaa(1)-Pro(2) peptide bonds as Xaa-Ala peptide bonds are unlikely to adopt the cis isomer, and examination of spectra from a library of 58 peptides indicates that ~80% of sequences show this effect. A simple mechanism suggesting that the barrier between the cis- and trans-proline forms is lowered because of low steric impedance is proposed. This observation may have interesting biological implications as well, and we note that a number of biologically active peptides have penultimate proline residues.