Theoretical equations of normal-pulse voltammetric current of the electron transfer (ET) across oil (O)/water(W) interface have been derived under certain, reasonable assumptions. These theoretical equations are considered to be useful to investigate the reaction mechanism whether it is the heterogeneous ET at O/W interface or the homogeneous ET associated with ion transfer across the interface. Extension of the theory to potential sweep voltammetry or cyclic voltammetry appears feasible.