The effect of intermolecular forces on isomeric equilibria (configurational changes in macromolecules are a special case) is investigated. The term corresponding to the second virial coefficient is derived but the method can easily be extended to higher terms. A significant and measurable effect is predicted for (a) isomeric equilibria involving atoms or small molecules or (b) configurational changes in macromolecules at a firsthyphen;order phase transition or critical point. A negligible effect is predicted for configurational changes in macromolecules otherwise. These conclusions apply to ``proper'' thermodynamic variables only; the endhyphen;tohyphen;end lengthrof a random coil is excluded.
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