The ketalization reaction of poly(vinyl alcohol) (PVA) by aromatic ketones, with dimethylsulfoxide (DMSO) as solvent, under the presence of an acidic catalyst in a homogeneous system, was carried out. The synthesis of poly(vinyl ketal) (PVKL) for the case of phenylacetone and benzylacetone was thus successfully performed, but for the case of acetophenone was performed only with a ketalization degree of a few molecular percent, and for the case of benzophenone it could not be performed at all. It seems that these different behaviors in synthesis are due to steric hindrance of the bulky side chain of ketones. The equilibrium constant at 40 degrees C was ca. 0.12 in the case of phenylacetone and benzylacetone, and the value is somewhat higher compared with that of aliphatic ketones, but somewhat lower compared with the case of cyclic ketones. Because the heat of reaction is 7.5 kcal/ mol in these two aromatic ketones, all ketalization reactions are considered to proceed by the same reaction mechanism. The rate of hydrolysis, contact angle, surface free energy, moisture regain, and water vapor permeability of PVKL films were measured. All results show that PVKL obtained from phenylacetone is nearly equal to PVKL obtained from methyl n-butyl ketone. However PVKL obtained from benzylacetone shows different behavior compared with other ketones, because the side chain of benzylacetone is flexible and bulky. The hydrophobicity of PVKL seems to depend upon the kind of the original ketones and the flexibility of the side chain.