Data are reviewed on the formation of third phase in the extraction of acti-nide(IV.VI) nitrates by neutral organophosphorus extractants, mainly tributyl phosphate. The data are critically evaluated and the effect of variables on the third phase formation is discussed. The variables are the concentrations of nitric acid and the extractant, temperature, the nature of diluent, addition of modifiers and the ionic strength of the aqueous phase. Also discussed are systems involving two extracted actinide ions.