The effect of sulfonation level and domain size of hydrophilic channels on humidity-induced phase transitions in poly(styrenesulfonate-methylbutylene) (PSS-PMB) block copolymers was studied as a function of the relative humidity (RH) and temperature of the surrounding air by a combination of water uptake measurements and in situ small-angle neutron scattering. The equality of the chemical potential of water in the gas and polymer phases was exploited to determine the change in the partial molar entropy of water at order-disorder transitions. PSS-PMB samples with 5 nm domain spacing exhibited a disorder-to-order transition with increasing temperature at Fixed RH, while the PSS-PMB samples with 7 nm domain spacing exhibited an order-to-disorder transition with increasing temperature at Fixed RH. There is evidence to Suggest that the disorder-to-order transition is driven by an increase in the partial molar entropy of the water molecules, while the order-to-disorder transition is driven by more familiar driving forces wherein entropic contributions stabilize the disordered phase.