This work shows a methodology to help flavorists and perfumers in the prediction of the headspace odor value for a ternary perfume mixture diluted in ethanol, based on the concept of perfumery ternary diagrams (PTD). In particular, this work takes into account the effect of liquid nonidealities in PTD, on the basis of the calculation of activity coefficients for all components in the liquid phase using the universal quasichemical functional group activity coefficients method. Using the chemical properties of the odorants and their odor threshold values in air, this technique allows a fast evaluation of all combinations of the three fragrant components for a fixed amount of solvent, and it eliminates some trial and error work usually done to choose a liquid mixture with a desired odor. Three examples of test mixtures representative of perfumes were used to illustrate these differences in PTD when considering the liquid as either an ideal mixture or a nonideal mixture. These systems are (1) benzaldehyde, phenylethanol, eugenol, and ethanol; (2) alpha-pinene, linalool, vanillin, and ethanol; and (3) limonene, geraniol, tonalide, and ethanol. The effect of the ethanol content in each ternary system was also studied.