Chemical structure of the top Al/C-60 interface in Al/C-60/ITO sandwich structures (Al is the thermally deposited top aluminum layer, C-60 is the thermally deposited fullerene layer, ITO is the double indiumtin oxide, the role of the substrate is played by Lavsan (polyethylene terephthalate), or glass) is studied by time-of-flight secondary mass spectrometry (ToF-SIMS) with depth profiling. The study is stimulated by the recently found specific features of the photovoltaic effect in fullerene-containing sandwich structures on glass or polymer substrates. It is found that the chemical composition of the top Al/C-60 interface is not the same on different substrates. This leads to differences in the photovoltaic conversion parameters for more complex thin-film structures with a molecular heterojunction.