silver dithizonates (Hg(HDz)2 and Ag(HDz)) were followed spectrophotometrically at high pressures and in various solvents. From the pressure dependence of the rate, the activationvolumes for the isomerization of Hg(HDz)2 in aprotic solvents were estimated to be about 6 cm3 mol−1, and were compared with those for Ag(HDz). The observed positive values of the activationvolumes have been explained in terms