The influence of carbon dioxide sorption on the closed- loop phase diagram of polystyrene- blockpoly( n- pentyl methacrylate) copolymers, PS- b- PnPMA, was investigated. Differential dilation of the constituent blocks was used to induce microphase separation of the PS-b- PnPMA. With increasing carbon dioxide loading an appreciable expansion in the size of the closed loop was found. At a carbon dioxide activity of 0.1 the upper order- to- disorder transition temperature ( UODT) was elevated by more than 20 C, whereas the lower disordertoorder transition temperature ( LDOT) was depressed by the same magnitude. This arises from the entropic nature of the closed- loop miscibility gap in this weakly interacting system, where microphase separation is driven by disparity in free volumes between dissimilar segments of the chain.