The influence of carbon dioxide (CO2) sorption on the phase behavior of two polystyrene-block-poly(n-alkyl methacrylate) copolymers was studied. One, polystyrene-block-poly(n-hexyl methacrylate), P(S-b-nHMA), exhibits an order-disorder transition (ODT), whereas the other, polystyrene-block-poly(n-butyl methacrylate), P(S-b-nBMA), exhibits a lower disorder-order transition (LDOT). CO2 sorption increases miscibility of the segments in P(S-b-nHMA) slightly: the ODT is depressed by less than 7 degreesC at a CO2 fluid density of 0.25 g/cm(3), which corresponds to 7 vol % dilation of the copolymer with CO2 at the conditions studied. In contrast, CO2 sorption decreased the miscibility of P(S-b-nBMA) markedly: the LDOT was depressed by more than 70 degreesC at densities < 0.06 g/cm(3), which corresponds to less than 3 vol % sorption Of CO2. Unlike P(S-b-nHMA), ordering transitions in CO2-dilated P(S-b-nBMA) exhibit a pronounced thermal hysteresis that increases with increasing volume fraction of sorbed diluent. The hysteresis is a consequence of the sensitivity of the LDOT system to differences in CO2 sorption between the ordered and disordered states, as evidenced by neutron reflectivity measurements. The difference in the effect of CO2 sorption on the phase behavior of the copolymers is attributed to the different nature of the transitions. The entropically driven LDOT is depressed by differential dilation of the copolymer domains, which increases both the compressibility of the system and disparities in compressibility between the blocks. In contrast, the enthalpically driven ODT is depressed by the screening of segmental interactions by CO2 and is less sensitive to compressibility.