A rigorous kinetic justification is given of the expression for interatomic tunnelling currents introduced in our earlier work [Stuchebrukhov, J. Chem. Phys. 105, 10819 (1996)] for the analysis of long-distance electron transfer reactions when a nonorthogonal atomic basis set is used for the description of the medium intervening between donor and acceptor. The analysis is based on the kinetic equations describing the evolution of the Mulliken populations of the atomic states in the medium during the tunnelling transition of a long-distance electron transfer reaction. (C) 1997 American Institute of Physics.