Several factors affecting the rate of heterogeneous electron transfer (ET) reactions at the nonpolarized interface between two immiscible electrolyte solutions (ITIES) were investigated by scanning electrochemical microscopy (SECM). The reactions between zinc porphyrin in organic phase and aqueous Ru(CN)(6)(3/4-) or Fe(CN)(6)(3/4-) redox species, which have previously served as model systems for probing the potential dependence of the ET rate constant, were used to investigate the effects of adsorption of a reactant and a potential-determining ion on ET rate at the interfaces between water and different organic solvents. It was demonstrated that the rate constant of an inter-facial ET can be either potential-independent or potential-dependent under different experimental conditions. The same model experimental systems were used to probe the kinetics of long-range ET across a monolayer of phospholipid adsorbed at the ITIES. The results obtained with a family of phosphatidylserine lipids are compared to those previously obtained with synthetic phosphatidylcholine lipids. The extent of blocking effect of adsorbed lipids on interfacial ET was investigated at different pHs and also in the presence of calcium ions in the aqueous phase. The micrometer-sized domain formation in the monolayer was observed in the presence of divalent cations.