THERE is a difficulty which can occur regarding the interpretation of heats derived from Clapeyron's equation applied to phase equilibria involving clathrates. This difficulty is not thermodynamic in origin but arises from the non-stoicheiometry of the phases. Before this non-stoicheiometry was realized, heats evaluated using Clapeyron's equation were considered to refer to fully stoicheiometric reactions, for example: the values 1 and 17 here being regarded as fixed integers. However, equation 1 should be formulated as: where x is less than 1, has not a fixed value but may vary with P and T, was unknown to earlier experimenters and indeed in this and many analogous systems is still unknown. x could vary from one experimenter to another according to the physical conditions employed. The heats derived have therefore referred to uncompleted reaction equations and so have an uncertain meaning1, until x is determined at the same time as the heat of reaction. Fortunately, x in many cases does not differ greatly from an integer, so that uncertainties resulting from non-stoicheiometry are not large.