The activities of TiC and TiN in Ti(CN) solid solutions can be calculated by means of an integration method. In order to obtain accurate activities for a given set of data, two approaches were evaluated. A linear regression with one curve was found to be more proper than a two-curve approach. The activity exhibited a strong negative deviation from ideal solution behavior. The free energy of formation was obtained at given temperatures using the activity values. Isothermal curves of the free energy of formation with respect to composition had a minimum value near Ti(C0.3N0.7) and Ti(C0.6N0.4) at 1700 and 2100 K, respectively. The investigation shows that the Ti(CN) solid solution cannot be treated as a regular solution. Thus, a subregular solution model was introduced to describe the temperature dependence of free energy.