The contribution of diffusion to the exciplex formation of the 1,2-benzanthracene (BZAN)/dimethylaniline (DMA) and pyrene (PY)/DMA systems in methycyclohexane at pressures up to 400 MPa was investigated. The rate constant, ks, for the exciplex formation, which was determined by the analysis of the rise and decay carve of the exciplex emission, decreased significantly with increasing pressure. The apparent activation volume for k(3), Delta V3(not equal), was 14.9 and 18.1 cm(3)/mol for BZAN/DMA and PY/DMA, respectively. For comparison, the rate constant, k(q), for the fluorescence quenching by carbon tetrabromide (CBr4) of BZAN and-PY that is believed to be fully diffusion-controlled was also measured as a function of pressure. The apparent activation volume for k(q), Delta V-q(not equal), was 17.9 and 20.1 cm(3)/mol for BZAN/CBr4 and PY/CBr4, respectively. These results are interpreted by assuming an encounter complex formed between the donor and acceptor pairs, and it was concluded that the exciplex formation reaction studied in this work is not fully diffusion controlled but competes with a diffusion process of the donor and acceptor molecules that is expressed by a modified Debye equation. It was also concluded that the quenching by CBr4 is not fully diffusion controlled at the lower pressure range. The bimolecular rate constant, k(bim) (= k(c)k(diff)/k(-diff)), for the exciplex formation was estimated to be 1.5 x 10(10) M(-1)s(-1) for BZAN/DMA and 1.9 x 10(10) M(-1)s(-1) for PY/DMA. The activation volumes for the various rate processes also are discussed.