PURPOSE: To obtain a 1,4-butanediol monoacetal convertible into butanediol as a raw material for polyesters by reaction of a propenal acetal with a synthetic gas in the presence of a rhodium catalyst using a triphenylphosphine derivative as reaction promoter.

CONSTITUTION: The 1,4-butanediol monoacetal can be obtained by reaction of a 2-propenal acetal with a synthetic gas in the presence of a rhodium catalyst using, as reaction promoter, a triphenylphosphine derivative bearing, on each aromatic ring, one to three such substituent(s) as to exceed zero in Hammett's substituent constant at 3-, 4- and/or 5-site(s) on the respective aromatic rings.