The protonation of the peptide bond (GO-NH) in an amphiphilic compound, the decyl N-dodecanoyl-sarcosylglycyl ester (DSG), has been studied in dimethyl sulfoxide (DMSO) acidified with trifluoromethanesulfonic acid (CF3SO3H). The surface-active properties of DSG (in DMSO) are however not accompanied by extensive micellization, as shown by diffusion NMR measurements combined with viscosity data. Protonation rates show clearly different kinetic laws depending on the concentration range studied (0.01-0.13 and 0.3-0.4 mol dm(-3)) and the Z-E stereoisomerism of DSG. At lower concentrations, proton transfers at 25 and 45 degrees C classically involve the solvated proton DMSO...H+ : k(H)(25 degrees C) = 7.7 and 5.5 s(-1), Delta K-double dagger = 41.3 and 37.0 kJ mol(-1), and Delta S-double dagger = -83.5 and -100.5 J mol(-1) K-1 for the cis and trans isomers, respectively. At higher concentrations and 45 degrees C, additional mechanisms are required, involving the N- and/or O-protonated peptide as proton carrier.