A predictive equation for the influence of chain ends on glass‐transition temperatures is derived from a thermodynamic theory for the compositional variation ofTg. This relation is found to give excellent agreement with results for the effect in polystyrene, and produces a convenient and accurate first approximation. Secondary approximations to the principal equation are formally identical to the Ueberreiter‐Kanig and Fox‐Flory equations for the variation ofTgwith molecular weight.