Selective 1,4-addition of water to butadiene monoxide can be achieved in a liquid phase catalytic process using ultrastable Y zeolite as Bronsted acid, potassium iodide, and an ether solvent. The reaction mechanism could be elucidated based on the occurrence of intermediate reaction products. Iodide is added to the terminal unsaturated carbon atom of a protonated butadiene monoxide molecule. The strong solvent effect observed can be rationalised based on the nucleophilicity of the iodide depending on the basicity of the ether function. With USY zeolite, the 2-butene-1,4-diol selectivity reaches 73.9% at 100% conversion when using 1,2-dimethoxyethane solvent, 2,5-dihydrofuran being the main byproduct. With this catalytic system followed by an additional hydrogenation step, butadiene monoxide can be selectively converted into 1,4-butanediol. (C) 1998 Academic Press.