Rate constants for complex formation between the positronium (Ps)-atom and the weak acceptor (Ac) nitrobenzene in water have been measured by us at various temperatures with particular emphasis on the slopes of the Arrhenius plot on either side of the observed turn-over point. Taken together with the temperature dependence of the surface tension and the viscosity of water, our data enable us to discuss the implications of our experimental findings as to the tenability of a proposed underlying reaction mechanism.