Peroxynitrous acid, an inorganic toxin of biological importance, acts both as an oxidizing and a nitrating agent during its conversion to nitric acid. In discussions of the mechanism of this conversion, activation volumes have been invoked to distinguish between possible mechanisms, viz., homolysis of the O-O bond versus rotation via the N-O bond of peroxynitrous acid. A reinvestigation of the activation volume for the conversion of peroxynitrous acid to nitric acid by high-pressure stopped-flow spectrophotometry yielded an average value of 6.9 +/- 0.9 cm(3) mol(-1) at 25 degreesC. Activation volumes currently cited in the literature for this process range from 6 to 10 cm(3) mol(-1) in the temperature range 18-25 degreesC. Such moderately positive values do not support a definite conclusion regarding the mechanism of the conversion.