The kinetics of the outer-sphere electron-transfer reaction between promazine (dimethyl-(3-phenothiazin-10-yl-propyl)-amine) and hexaaquairon(III) was studied using a high-pressure stopped-flow technique. The effect of pressure (over the range 0.1-130 MPa at 25 degreesC and ionic strength 1.0 M) on the reaction rate in aqueous perchloric acid solution resulted in volumes of activation of -6.2 +/- 0.4 and -12.5 +/- 0.5 cm(3) mol(-1) for the forward and reverse processes, respectively. The effect of pressure on the overall equilibrium constant revealed a reaction volume of +5.0 +/- 0.2 cm(3) mol(-1). The reported volume profile reveals mechanistic information on the electron-transfer process in terms of volume changes along the reaction coordinate. The volume of activation for the promazine/promazine(+.) self-exchange reaction was calculated on the basis of the Marcus cross relationship.