Electrochemical surface plasmon resonance (ESPR) has been employed as a probe of redox reactions at the interface between gold (Au) and ionic liquid (IL). Two redox couples, ferrocene (Fc)/Fc(+) and (ferrocenylmethyl) trimethylammonium (FTA(+))/FTA(2+), and two ILs, trioctylmethylammonium bis(trifluoromethanesulfonyl) amide ([TOMA(+)][C1C1N-]) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(4)mim(+)] [C1C1N-]) have been studied. In the ESPR measurements, the shift of the SPR angle has been recorded simultaneously with cyclic voltammogram (CV). It has been revealed that the SPR angle shift reproduces CV when the former is semi-differentiated or the latter is semi-integrated, and, therefore, that the ESPR response probes the surface concentration of redox couples at the IL vertical bar Au interface. Among the four combination of the two redox couples and the two 'Ls, the case for Fc/Fc(+) in [TOMA(+)][C1C1N-] shows significantly greater ESPR response than the other three cases. A model has been established for the relationship between the SPR angle shift and the surface concentration of redox species. The model predicts that the SPR angle shift becomes pronounced with increasing D-R/D-O, the diffusion coefficient ratio of the reduced (R) and oxidized (O) species when the reduced species is initially dissolved in the IL as is the case for the present study. Significantly greater D-R /D-O for Fc/Fc(+) in [TOMA(+)][C1C1N-] than the three other cases has been confirmed from the D-R and D(O )measurements by CV with a microdisk electrode. The trend of the measured D-R and D-O values agrees with recent findings by researchers that small neutral solutes in ILs composed of large ions diffuse fast beyond the prediction of the Stokes-Einstein relation.