Primary kinetic isotope effects (KIEs), k(H)/k(D), have been determined spectrophotometrically for the reactions of NAD(+) analogues (acridinium ions, 1a-e(+), and quinolinium ion, 2(+)) with heteroaromatic compounds such as 3-methyl-2-phenylbenzothiazoline, 3H(D), and 1,3-dimethyl-2-phenylbenzimidazoline, 4H(D) in a mixed solvent: containing four parts 2-propanol and one part water at 25.0 +/- 0.1 degrees C. The KTEs decrease from 6.24 to 3.93 as the equilibrium constant, K, is increased from about 1 to 10(12) by the structural variation in the hydride acceptor. The Marcus theory of atom transfer in a linear, triatomic model of the reaction, with tunneling, can explain the variation of KIE with K. The Marcus theory is based oil a model involving no high-energy intermediates, leading to a one-step mechanism. The present system satisfies this condition.