AbstractA method of plotting the logarithms of the activity coefficients of each component in a ternary system is described which takes advantage of information on the three binary systems. For a ternary system A, B, C the values of log γA againstxc, log γB againstxc, and log γc againstxB are plotted for a constant value ofxB/(xB+xc), wherexA,xBandxcare the mole fractions of the three components in the liquid. If the whole ternary diagram is to be explored these plots are carried out for a series of values of the parameterxB/(xB+xc). A method of interpolation is described and thermodynamic tests for the consistency of the resulting plots are developed. The use of the procedure is illustrated by application to published information on the system acetonitrile (methyl cyanide)‐trichloroethylene