From a chemical point of view methanol is one of the closest analogues of water. Consistent with this idea EPR spectroscopy studies have shown that methanol binds at—or at least very close to—the Mn₄O _x Ca cluster of photosystem II (PSII). In contrast, Clark-type oxygen rate measurements demonstrate that the O₂ evolving activity of PSII is surprisingly unaffected by methanol concentrations of up to 10%. Here we study for the first time in detail the effect of methanol on photosynthetic water-splitting by employing a Joliot-type bare platinum electrode. We demonstrate a linear dependence of the miss parameter for S _i state advancement on the methanol concentrations in the range of 0–10% (v/v). This finding is consistent with the idea that methanol binds in PSII with similar affinity as water to one or both substrate binding sites at the Mn₄O _x Ca cluster. The possibility is discussed that the two substrate water molecules bind at different stages of the cycle, one during the S₄ → S₀ and the other during the S₂ → S₃ transition.