Abstract: A process for preparing malonic acid from the hydrolysis of cyanoacetic acid in the presence of aqueous hydrochloric acid, the process conditions being such that they allow high yields in malonic acid product. The product of said acid hydrolysis is a mixture of malonic acid and ammonium chloride by-product; the mixture is concentrated, the ammonium chloride by-product is separated by dissolution with an oxygen solvent and isolated. Alternatively, said concentrated mixture is dissolved with a primary or secondary esterifying alcohol in C1-C10, the resulting mixture being esterified, while the ester product is purified by distillation under reduced pressure, the molar yields and purity of the ester product being high.