Electrochemistry of cytochrome c (cyt c) immobilized on a cardiolipin (CL) / phosphatidylcholine (PC) film supported on a glassy carbon electrode was investigated using variable-frequency AC voltammetry. At low ionic strength, we observed two redox-active subpopulations characterized by distinct values of potential (E1/2) and electron transfer rate constant (kET). At high ionic strength, only one subpopulation was detected, consistent with the existence of very stable cyt c – CL adducts, most probably formed by hydrophobic interactions between the protein and the fatty acid (FA) chains carried by CL. This subpopulation exhibits a comparatively high kET value (> 300 s−1) apparently changing with the structure of the FA chains of CL, i.e. 18:2(n-6) or 14:0. Our study suggests that electrochemistry can be a useful technique for probing protein − lipid interactions, and more particularly the role played by the specific structure of the FA chains of CL on cyt c binding.