The decay time of the luminescence from mono-molecular and excimer states in very thin tetraphenylporphyrin (H2TPP) films evaporated on SnO2 substrates has been measured in situ in a pressure of 2 × 10-6 Pa using a picosecond dye laser at 100 K. The decay time of the 665 nm luminescence in H 2TPP on a highly conductive SnO2 substrate (σ = 77 Ω-1 cm-1 is 800 ps in films with a thickness above 25 Å and decreases with decreasing thickness to 170 ps at 7.5 Å. Contrarily, the decay time for films on low conductive SnO2 (σ = 4.5 × 10-3 Ω-1 cm-1) is independent of film thickness. The luminescence quenching ratio calculated from the decay time coincides well with that from the relative luminescence intensity. This coincidence leads to the conclusion that luminescence quenching occurs through the mono-molecular excited state or the excimer state. We propose the following mechanism to explain these experimental results. First, a conduction electron of the highly conductive SnO2 substrate recombines with a hole of the H2TPP excimers resulting in a charge transfer state. Second, an electron is transferred from the excimer state of H2TPP to the conduction band.