On comparing the last two equations we conclude that γi is a partial molar property with respect to   Thus, we have:

It follows from the definition of activity coefficient (eqn. 6.163) that for an ideal solution its value is unity for all species as G^E = 0. For a non-ideal solution, however, it may be either greater or less than unity, the larger the departure from unity the greater the non-ideality of the solution. The derivatives of the activity coefficient with respect to pressure and temperature can be correlated to the partial molar excess volume and enthalpy respectively.

Using eqn. 6.167 and 6.168 in conjunction with 6.173/6.174 the following results are obtained: