A method for the conversion of esters to carboxylic acids has been reported. Reaction of methyl o-methoxybenzoate with N-(trimethylsilyl)diethylamine and methyl iodide, followed by hydrolysis, afforded o-methoxybenzoic acid with a 94% yield based on 96% conversion. Methyl esters of not only aromatic acids but also aliphatic acids were converted to the corresponding acids with high yields. A combination of N-(trimethylsilyl)dimethylamine and methyl iodide was also effective to give p-methylbenzoic acid with a 85% yield based on 90% conversion from the corresponding methyl ester.