Phase separation behavior has been studied in aqueous solutions of partially butyralized poly(vinyl alcohol) (BuPVA) with various degrees of butyralization xBu and various molecular weights. It is found that these systems exhibit both upper and lower critical solution temperatures. The theta temperatures θLCST, evaluated by means of Shultz–Flory plots, are found to be 25.1, 23.3, and 14.4°C for BuPVAs with xBu of 7.5, 9.9, and 12.7 mol %, respectively. The unperturbed dimension 〈R2〉0/M is evaluated as ca. 1.2 × 10−16 A2 in the above range of xBu from viscosity measurement at θLCST. Properties of the BuPVA solutions are compared with those of other PVA copolymers.