The influence of high pressure on the diastereoselectivity of the intermolecular all-carbon Diels - Alder reaction of the phenylbutadienes 1a-c with the dicyanoethylenes 2a-d to give the cyclohexenes 3-8 is described. The differences in activation volume, aav, for the two pathways leading to cis and trans diastereomers range from - (0.7 +/- 0.8) to - (6.4 +/- 0.6) cm(3) mol(-1), indicating a pressure-induced increase of diastereoselectivity in favour of the cis adducts 3a-d, 5a-d and 7b-d.