NMR studies and theoretical calculations have been performed on model peptides Ac-Ser(Psi Pro)-NHMe, (S,S)Ac-Ser(Psi(H,CF3)Pro)-NHMe, and (R,S)Ac-Ser(Psi(CF3,H)pro)-NHMe. Their thermodynamic and kinetic features have been analyzed in chloroform, DMSO, and water, allowing a precise description of their conformational properties. We found that trifluoromethyl C-delta-substitutions of oxazolidine-based pseudoprolines can strongly influence the cis-trans rotational barriers with only moderate effects on the cis/trans population ratio. In CHCl3, the configuration of the CF3-C-delta entirely controls the psi-dihedral angle, allowing the stabilization of gamma-turn-like or PPI/PPII-like backbone conformations. Moreover, in water and DMSO, this C-delta-configuration can be used to efficiently constrain the ring puckering without affecting the cis/trans population ratio. Theoretical calculations have ascertained the electronic and geometric properties induced by the trifluoromethyl substituent and provided a rational understanding of the NMR observations.