Abstract Examination of the inhibition of positronium formation in aqueous solutions, by a wider range of solutes shows that inhibiting power is not determined only by the rate constant for the reaction of the solute with aquated electrons, nor by the ability of the solute to inhibit formation of aquated electrons in fast pulse radiolysis. The inhibition by selected solutes in different solvents has been investigated. Inhibition is not necessarily greater in the less polar solvents. The data suggest that the spur model for positronium formation in solutions is probably only one of the mechanisms involved. Both inhibition and quenching reactions of solutes are sensitive to the structure of the solute; the maleate and fumarate anions showing quantitatively very different behaviour.