Electron transfer (ET) reactions in ionic liquid (IL)vertical bar organic solvent (1,2-dichloroethane, DCE) and IL vertical bar water mixtures were investigated using a Pt disk ultramicroelectrode (UME) along with ferrocene (Fc) and ferrocenemethanol (FcCH(2)OH) redox probes as electroactive species dissolved in the respective mixtures. The IL utilized was tributylmethylphosphonium methyl sulfate (P4441CH3SO4). The diffusion coefficient of each redox species was determined at each incremental increase of DCE or water to the IL using a chronoamperometric technique that is concentration independent. The IL vertical bar DCE mixture exhibited little change in the Fc diffusion coefficient, D-Fc, up to a DCE mole fraction (chi(DCE)) of 0.5; the observed value, 2.0 x 10(-)8 cm(2) s(-1), agrees well with that typically reported for ILs in the literature. After which, the DFc quickly rose to a value commonly found in conventional molecular solvents, 1.3 x 10(-5) cm(2) s(-1) (at chi(DCE) = 0.8). An analogous result was not observed for IL vertical bar water mixtures using FcCH(2)OH, such that D-FcCH2OH varied from 0.2 to 1.2 x 10(-9) cm(2).s(-1) at a chi H2O of 0 to 0.8. It was proposed that a large increase in the D-Fc in the IL vertical bar DCE mixture versus DFcCH(2)OH in the IL vertical bar water series was owing to P4441CH3SO4's more hydrophobic character. Its hydrophobicity was quantified by measuring the formal ion transfer potentials of the IL component ions at a water vertical bar DCE immiscible interface.