The present invention provides a process for the preparation of optically active carboxylic acids of the formula I* or II* ##STR1## in which X is an oxygen or sulfur atom and n is 1 or 2. The process comprises reacting a racemic carboxylic acid I or II or its derivatives with an optically active 2-amino-carboxylic acid ester to give the diastereomeric carboxylic acid amides, separating the diastereomers and, after cleavage of the amide bond, isolating the optically active carboxylic acids of the general formula I* or II*. The invention also provides certain novel optically active carboxylic acid amides and a tetrahydrothiopyran-2-carboxylic acid.