The unavoidable presence of impurities in metals generally leads to an offset of the fusion interval towards higher, or lower temperatures depending on the nature of the (metal + impurity) phase behaviour. In this paper, the effect of trace impurities on metal fixed points is considered both theoretically and experimentally. A thermodynamic analysis of the effect of impurities on the fractional fusion behaviour, and on the heat capacity in the fusion range shows that all impurities can be classified into three categories: eutectic-type, peritectic-type, and nearly-degenerate type. The consequences of impurities in metals that are used for fixed-point determinations are discussed in the light of the accuracy needed in the temperature determinations. The theoretical considerations are supplemented by an experimental study of the fusion characteristics of high-purity tin samples contaminated with a Bi mole fraction of 271 . 10(-6) and Sb mole fractions of (113 and 370) 10(-6). The {(1 - x)Sn + xBi} system exemplifies a eutectic-type behaviour, and the {(1 - x)Sn + xSb} system a peritectic-type behaviour. The influence of other impurities is also briefly considered. The evaluation of the purity of a compound from fractional fusion characteristics is discussed.