A decomposition of the entropy of dilute ternary polymer–solvent–cosolvent systems into pair terms is proposed. The variation of each term and its contribution to the total entropy in a coil–globule–coil transition is quantified by molecular dynamics simulations. Contrary to intuition, the modifications of the intramolecular polymer entropy are much smaller than the ones in the polymer–solvent and polymer–cosolvent entropy. For each solvent–cosolvent concentration, we identify the leading entropy term. Furthermore, we relate the modifications of all pair entropies to the occurring changes in the polymer structure.