The antiferroelectric A2/a ↔ P21/a phase transition in titanite may be described using a thermodynamic model where the principal contribution to the excess entropy is assumed to be configurational rather than vibrational; G = A/2 Q2 + B/4 Q4 + λT [(1 + Q) ln (1 + Q) + (1 − Q) ln (1 − Q)]. Such a model is likely to be a valid description of a phase transition where strain effects are large enough to maintain mean field behavior, but not so large that vibrational effects dominate in the excess entropy. Best-fit parameters for A/λ and B/λ are determined from X-ray measurements of the order parameter. The magnitudes of the three parameters are then determined using calorimetric data, with the results A = −337.6 J/mol, B = −112.5 J/mol, λ = 0.34 J/mol-K. The model is compared with measurements of dielectric susceptibility, and is found to give good, but not perfect, agreement with experiment. The excess entropy associated with the transition is far smaller than expected for a simple order-disorder model. This is interpreted as evidence for significant short-range order above TC.