[en] Kinetic isotope effects have been determined for the reactions of 4-nitrophenylnitromethane (4-NPNM) and its deuteriated analogue with various neutral nitrogen bases in anisole solution. The reaction is a proton-transfer leading to an ion-pair. The values of the isotopic rate ratio ksup(H)/ksup(D) at 250C are all large (> 18) and are attributable to tunnelling. The values for two bases containing the grouping NH = C =, namely tetramethylguanidine, NH = C[N(CH3)2]2, and N,N-diethylbenzamidine, NH = C(C6H5) N(C2H5)2 are markedly larger than those for the tertiary amines, quinuclidine and triethylamine. The reasons are discussed. (author)