Lattice vibrations are incorporated into the Gibbs‐DiMarzio configurational entropy theory of glasses. A comparison is made with data compiled by O’Reilly. The resulting theory predicts the specific‐heat discontinuity at the glass transition to within 20%. No adjustable parameters are involved; only the chemical structure need be known. The specific‐heat discontinuity has three parts. The formula in customary notation is &Dgr;cp=R(&Dgr;&egr;/kTg)2 f(1−f) +RTg&Dgr;&agr; (4−Tg&Dgr;&agr;/0.06) +0.5Tg&Dgr;&agr;cp(Tg−). The first part is a configurational term arising from shape changes of the molecules (?50% of total). The second part is a configurational term arising from volume expansion (?30% of total). Finally there is a vibrational contribution arising from the change of the force constants (or characteristics frequencies) with temperature (?20% of total).