A new conceptual basis is outlined for chemical reactions between positronium and ionic or neutral species in liquids. It is first shown that the positronium formation fraction in pure liquids depends not only on the dissociation energy of the medium (Oré-gap theory) but also on the threshold, I_A–I_P, where I_A and I_P are the ionisation potentials of the liquid and positronium. The energy regions in which positronium reactions may occur are defined in terms of ionisation potentials and bond strengths of the constituents of the liquid and of positronium. Special features of positronium reactions are described, and general conditions and examples are given of oxidation of Ps by electron transfer, compound formation, and double decomposition, and of reduction. ‘Anomalous’ features of earlier experimental data and recent experiments are satisfactorily explained in terms of the new concepts. These concepts are then applied to describe the conditions for reactions in mixtures. The effects are additive except when complexes are formed.

A simple mathematical analysis is given to predict the effects on the experimental position annihilation lifetime data which depend on ortho–para conversion, pick-off, and compound formation. Experimental data are used, when available, to check the predictions.