A simple expression is given for the isothermal reversible work of formation of a vapor lens at a liquid-liquid interface, and necessary conditions derived in terms of the spreading coefficients of the two liquids on each other. The magnitude of the interfacial tension is found to be crucial to the satisfaction of these conditions, and an approximate expression of Fowkes enables an estimate of a criterion in terms of the contributions of the London dispersion forces to the interfacial tensions. A two-step mechanism for the UO2-Na system is also postulated.