The phase transitions of polystyrene-block-poly(n-pentyl methacrylate) (PS-PnPMA) have been investigated by temperature-dependent infrared spectroscopy with two-dimensional correlation spectroscopy. PS-PnPMA was shown to exhibit a closed-loop type of phase behavior, where, upon heating, a disorder-to-order transition (LDOT) was found at lower temperature and an upper order-to-disorder transition (UODT) was observed at higher temperature. Sudden changes in the absorbance (A) and maxima in the slope dA/dT were observed at the transition temperatures. In the disordered state below the LDOT, the intensity of the conformational band at 1255 cm(-1) was greater than that at 1274 cm(-1), whereas the reverse was found in the disordered state above the UODT. This suggests that the conformation of the C-C-O group in the PnPMA chain (and thus the directional enthalpic gain) is different in the two disordered states and, therefore, that the driving force to induce the disordered state at lower temperatures is different from that at higher temperatures.