[en] The reaction of the deuterated analogue of 4-nitrophenylnitromethane with tetramethylguanidine in toluene shows complex kinetics compared to the corresponding proton-transfer reaction. The complex kinetics can be accounted for by an H - D exchange mechanism. Comparison of experimental data and data calculated from the assumed reaction mechanism suggests a kinetic isotope effect of about 11 at 30 deg C which is confirmed in experiments using deuterated tetramethylguanidine. The results are not in agreement with previously reported values and suggest that the isotope effect and degree of tunnelling is much less than previously supposed. (author)