A study is reported of the influence of pD upon the 220 MHz proton n.m.r. spectrum of the dipeptide glycylsarcosine (Gly-Sar) in D2O at 23 °C. Variation of pD produces characteristic changes in chemical shifts for Gly-Sar and an analysis of the pD–chemical shift curves shows that the carboxyl pK differs significantly in cis and trans conformers, having values of 3.01 ± 0.03 and 3.39 ± 0.03, respectively. From a consideration of molecular models and theoretical calculations, it is concluded that the pK difference arises from a closer juxtaposition of terminal carboxyl and amino groups in the cis isomer.