We have demonstrated a novel application of hybrid two-dimensional (2D) correlation spectroscopy in exploring the reversibility of phase separation (or transitions). Pressure-induced phase separation and temperature-induced phase transitions have been studied for poly (N-isopropylacrylamide) (PNiPA) and poly(N-isopropylmethacrylamide) (PNiPMA) in aqueous solutions using Fourier transform infrared (IR) and hybrid 2D correlation spectroscopy. Here we show that pressure-dependent sample-sample (SS) hybrid 2D correlation IR spectroscopy reveals for PNiPA a reversible phase separation with a differential rate of concentration change and separation pressure and a reversible phase separation with similar dynamics for PNiPMA, but involving irreversible hydrogen/deuterium exchange. Subtle differences in concentration dynamics have been revealed clearly by the hybrid SS 2D IR correlation analysis; these include the minor irreversibility of amide I in PNiPA, resulting from the loss of a very few coil populations, and in PNiPMA, resulting from the irreversible HID exchange. Such events are not apparent from the transition plots constructed on the basis of one-dimensional data analysis.