Equations for the calculation of the Kirkwood–Buff integrals (KBI) for the components of a ternary system from thermodynamic data are presented and applied to a number of mixtures of non-electrolytes. The mixtures comprise substances which may develope strong mutual interactions due to e.g. permanent dipoles and hydrogen bonds; they are: (I) water–propan-1-ol–N,N-dimethylformamide (DMF), (II) water–ethane-1,2-diol–acetonitrile, (III) water–ethane-1,2-diol–ethanol, (IV) ethanol–trichloromethane–1,4-dioxane; the system (V), water–propanol–urea, previously studied is also discussed. The observed KBIs have been correlated to interactions among functional groups and to the structure of the solvent components. Moreover, a preferential solvation coefficient of a given compound with respect to the other two is given a different definition to that of previous authors and calculated; its change with changing concentration of solvating components is examined. These quantities have proved to be useful indicators of the predominant types of interactions in various concentration regions.

In systems I and V we compare the effect of the addition of either DMF or urea to the binary mixture, water–propanol, which is characterised by large positive values of the like (i–i) KBI. A markedly different behaviour is found: whereas urea causes a strong increase of the like KBIs, DMF has the opposite effect. Moreover, urea is mostly solvated by water over the whole water–propanol mole fraction range, whilst DMF presents a sharp change of preference at ca. x_w= 0.8, being preferentially solvated by propanol in the water-rich region. The results from systems II and III, where the effect of the addition of either ethanol or acetonitrile to the mixture water–ethanediol is examined show, in particular, that the KBIs of ethanol and acetonitrile behave as if the mixture water–ethanediol were a single component. In system IV the observed KBI behaviour is related to the competition between hydrogen-bond and dipole–dipole type interactions.