Abstract: In a process for preparing alpha-amino-gamma-methylmercaptobutyronitrile by aminating, by means of ammonia, alpha-hydroxy-gamma-methylmercaptobutyronitrile at a temperature of 50.degree. C to 100.degree. C, the ammonia is accompanied by an amount of water corresponding to at least 1 mole per mole of ammonia.The molar NH.sub.3 /cyanhydrine ratio in the reaction medium is 2 to 10, and preferably 4 to 7, the H.sub.2 O/cyanhydrine ratio in the reaction medium is 4 to 20, and preferably 5 to 16, while H.sub.2 O/NH.sub.3 ratio is 1 to 3, and preferably 1.3 to 2. Advantageously the reaction medium is introduced into a piston-type tubular reactor which is maintained at a constant temperature, the dwelling time of the mixture in the heated zone being 1 to 30 minutes, and preferably 2 to 25 minutes, and the reaction medium is maintained under a pressure of 1 to 10 bars, and preferably a 4 to 8 bars.