Direct electrochemistry has been developed into a powerful tool for the analysis of soluble proteins and enzymes over the past two decades. In the present study, we have expanded the range of proteins to include a solubilized membrane protein, photosystem I. Taking into account the difficulties caused by the presence of detergent, we have shown it to be possible to obtain an electrochemical signal of the primary electron donor, a chlorophyll dimer called P-700, of photosystem I. A reduction potential of E-1/2 = 490 mV vs. SHE with a peak separation of DeltaE = 100 mV was found, in good agreement with literature values.