The effect of high pressures up to 400 MPa on the emission quenching of Coumarin 343 (C343) adsorbed on TiO2 nanocrystallites in methanol (MeOH) and N,N-dimethylformamide (DMF) solution was investigated. Tn MeOH, the quenching efficiency decreased as the pressure increased. This decrease was explained by the decrease of the adsorption strength of C343 onto TiO2 nanocrystallites. On the contrary, the efficiency in DMF increased with increase of pressure in spite of decrease of adsorption strength of C343 onto TiO2 under the high pressure condition, indicating enhancement in the electron transfer efficiency. The results imply that the high pressure technique can control the efficiency of photo-induced electron transfer systems.