Theory of normal-pulse voltammetric current of electron transfer (ET) reaction at organic solvent(O)/water(W) interface between a redox couple of A species, A(RED) and A(OX), in O phase and another redox couple of B species, B-OX and B-RED, in W phase has been discussed. A(RED) is supposed to be a neutral molecule and A(OX) a cation, whereas both B-RED and B-OX are supposed to be highly hydrophilic. Then, two kinds of the reaction mechanism, that is, (1) the heterogeneous ET reaction: A(RED)(O) + B-OX(W) = A(OX)(O) + B-RED(W) at the O/W interface producing the voltammetric current, and (2) the transfer of A(RED)(O) molecule from O to W phase across the interface, then the homogeneous ET reaction: A(RED)(W) + B-OX(W) = A(OX)(W) + B-RED(W) in W phase, followed by the transfer of A(OX)(W) cation from W to O phase producing the voltammetric current, are considered. Theoretical equations of normal-pulse voltammetric current of the ET reaction at O/W interface are derived under the assumptions (i) that the ET reaction is pseudo-monomolecular with respect to A species, (ii) that the partition of A(RED) molecule between O and W phases is highly biased to O phase, and (iii) that the rate constants of homogeneous ET reaction in W phase are sufficiently large. Application of the theoretical equations to investigate the mechanism of the ET reaction at O/W interface has been discussed. (c) 2006 Elsevier B.V. All rights reserved.