The cis/trans isomerization of peptides containing the pseudoproline (4R)-thiazolidine-4-carboxylic acid Cys(Psi(R1.R2) pro) is investigated from both an experimental and a theoretical point of view by NMR and DFT calculations. A series of Ac-Cys(Psi(R1.R2) pro)-OCH3 and Ac-CyS(Psi(R1.R2) pro)-NHCH3 peptides were prepared to assess the influence of the substitution at the C-2 position as well as of the amide following the thiazolidine residue. For each compound, the cis/trans ratio along with free energy, the puckering of the thiazolidine ring and the free rotational energy barrier are reported and discussed. We observe there is a pronounced effect of the C-2 substituents and of the chirality upon the cis/trans ratio with the population of the cis content in the order (2R)-Cys(Psi(CH3,H) pro) < (2S)-CyS(Psi(H,CH3) pro) < Cys(Psi(CH3,CH3) pro). This is interpreted as a consequence of the steric effects imposed by the 2-methylation. For the dimethyl-substituted compounds our results revealed a destabilization of the trans conformers along with a lowering of the rotational trans to cis barrier. We also demonstrate that the introduction of methyl substituents on the thiazolidine cycle reduces the autocatalysis effect, and in particular that no autocatalysis occurs for the dimethyl derivative. A good agreement is obtained between all our experimental observations and our DFT calculations. It rationalizes the main effects which tailor the cis/trans isomerization.