The invention relates to a process for the diastereoisomeric resolution of 5-methyltetrahydrofolic acid； it discloses also a crystalline amine salt of 5-methyltetrahydrofolic acid and is further directed to a process for the transformation of the single diastereoisomers of the 5-methyltetrahydrofolic amine salts obtained by the process of the invention into the corresponding free acid or into an alkaline earth metal salt. The invention provides a simple, economically advantageous and efficient process and allows to obtain stable and pure (6R) and ( 6S) -diastereoisomers of 5-methyltetrahydrofolic acid in a good yield and diastereoisomeric purity.
