For the vapor phase hydrolysis of a vinylidene-type dichloride into its corresponding carboxylic acid, several efficient catalysts were found, They are acidic mixed metal oxides: WO3-TiO2, WO3-ZrO2, and Nb2O5-TiO2. 1,1-Dichloro-3,3-dimethyl-1-butene (DDB) was effectively hydrolyzed over WO3(1)-TiO2(4) catalyst into 2,2-dimethylbutyric acid (DMBS), a key starting compound for a herbicide, as follows: conversion of DDB, 41%; selectivity to DMBS, 96.3%. The catalytic activity was proportional to the amount of Bronsted acidity with acid strength stronger than H-0 less than or equal to- 3.0.