Isomeric effects on the phase behavior of Polystyrene-blockpoly(pentyl methacrylate) copolymers were studied. Results show that when the molecular weight was judiciously controlled, PS-b-PneoPMA shows the closedloop type phase behavior similar to PS-b-PnPMA. PS-b-PneoPMA-H shows a precipitous drop in the birefringence signal at approx 278°C. The closed-loop phase behavior of PS-b-PneoPMA shows that at 162.5°C, the peak became sharp and narrow. A plot of inverse maximum intensity of SAXS profiles versus inverse temperature indicates temporal changes of storage modulus G' of PS-b-PneoPMA-B3. The molecular weight dependence of transition temperatures for PS-b-PisoPMA and PS-b-PneoPMA shows different molecular geometries of pentyl methacrylate block. The free volume for PneoPMA is larger than that for PnPMA because of the bulkier neopentyl chains compared with linear shape of n-pentyl chains.