Whether the secondary isotope effect of deuterium is base-strengthening (‘normal’, KH/KD>1) or acid-strengthening (‘inverse’, KH/KD<1) depends in some measure on the molecular environment; a strongly electron-demanding environment tends to enhance the inverse isotope effect relatively. The basic ionization constants of perdeuteriated pyridines show remarkably low values of KH/KD: 1·028, 1·027, 1·013, and 0·966 for the 4-amino-, 4-methoxy-, 4-chloro-, and 4-nitro-derivatives respectively. A base-weakening effect of deuterium is not necessarily evidence of hyperconjugation. These results do not appear to be due to changes in vibrational zero-point energies. A quasi-electronic isotope effect provides a satisfactory explanation: as the D–C bond has a lower inductomeric polarizability than the H–C bond it is less able to release electrons to meet a strong electron-demand.