The thermosensitive phase separation of poly(vinyl methyl ether) (PVME) aqueous solutions has been investigated using near-infrared spectroscopy in combination with two-dimensional correlation analysis, and a two-step phase separation mechanism during gradual heating has been established. Two-dimensional near-infrared (2D NIR) analysis results indicate that during this two-step process the dehydration of CH2 groups occurs earlier than that of CH3 groups. This result suggests that it is the change of the hydrophobic hydrocarbon chain conformation induced by heating that indirectly leads to the dehydration of the hydrophilic ether oxygen side groups.