A new inlet system for the application of the differential electrochemical mass spectroscopy to massive electrodes is presented. The main advantage of our electrochemical cell is the free access of the massive electrode surface, which allows a homogeneous illumination of the electrode/electrolyte interface as well as measurements in the presence of strong gas evolution (high current densities). Because the cell is directly connected to the high-vacuum chamber of the mass spectrometer unit, a good sensitivity is achieved with a relatively simple experimental setup. As an example for a photoelectrochemical system, p-InP(111) single crystals were investigated in the dark and under illumination during cathodic scans in 0.5 M H2SO4. Simultaneous to the hydrogen evolution, a photocorrosion can be observed via the mass signal of phosphine (PH3). Energy dispersed analysis of X-ray fluorescence and scanning electron microscopy measurements reveal the formation of metallic indium sites at the electrode surface due to the corrosion process. It is shown that the electrodeposition of platinum onto the surface of a bare p-InP(111) electrode leads to a catalytically enhanced hydrogen evolution whereas the corrosion process is drastically suppressed.