Preparation of 2-hydroxy-4-methylselenobutyric acid (i), comprises reaction of 3-methylselenoproprionaldehyde (ii) with an alkaline cyanide in the presence of an alkaline bisulfite salt in a protic polar solvent, to form 2-hydroxy-4-methylselenobutyronitrile (iii) and hydrolysis of (iii) in a strongly concentrated acid, under HEAT, in a protic polar solvent to form (i) and optionally transforming (i) into one of its salts after addition with an acid. Preparation of 2-hydroxy-4-methylselenobutyric acid (i), comprises reaction of 3-methylselenoproprionaldehyde (ii) with an alkaline cyanide of formula M +>CN - >, in the presence of an alkaline bisulfite salt of formula M +>HSO 3 - >, in a protic polar solvent, to form 2-hydroxy-4-methylselenobutyronitrile (iii) and hydrolysis of (iii) in a strongly concentrated acid, under HEAT, in a protic polar solvent to form (i) and optionally transforming (i) into one of its salts after addition with an acid. M: an alkali metal atom. Independent claims are included for: (1) preparation of 2-hydroxy-4-methylselenobutyric acid and 2-hydroxy-4-methylthiobutyric acid comprising reaction of 3-methylselenoproprionaldehyde and 3-methylthiopropionaldehyde with an alkaline cyanide (M +>CN - >), in the presence of an alkaline bisulfite salt (M +>HSO 3 - >) in a protic polar solvent, to form 2-hydroxy-4-methylselenobutyronitrile and 2-hydroxy-4-methylthiobutyronitrile followed by hydrolysis in an hot strong acid medium, in a protic polar solvent to form 2-hydroxy-4-methylselenobutyric acid and 2-hydroxy-4-methylthiobutyric acid, which can be optionally transformed into a mixture of their salts and where the ratio of the final compounds are fixed at the starting of the process the relative quantities of the starting compounds and maintained throughout the reaction (2) 2-hydroxy-4-methylselenobutyronitrile (iii) or its enantiomers and (3) nutritional composition comprising a mixture of 2-hydroxy-4-methylselenobutyric acid and 2-hydroxy-4-methy
