Abstract: A process for preparing .alpha.-L-aspartyl-L-phenylalanine methyl ester: ##STR1## in which: (a) the compound methyl N(.alpha.-L-aspartyl).alpha.-aminocinnamate, protected at the nitrogen, of formula: ##STR2## where R is a protector group at the nitrogen, is subjected to hydrogenation at the olefin bond by means of gaseous hydrogen in the presence of a hydrogenation catalyst, to give the compound of formula: ##STR3## where R has the aforesaid meaning, in the form of a mixture of .alpha.-L-aspartyl-L-phenylalanine methyl ester protected at the nitrogen, and .alpha.-L-aspartyl-D-phenylalanine methyl ester protected at the nitrogen;(b) the protector group is removed from the .alpha.-L-aspartyl-L-phenylalanine and .alpha.-L-aspartyl-D-phenylalanine methyl esters protected at the nitrogen;(c) the .alpha.-L-aspartyl-L-phenylalanine methyl ester is separated and recovered from the deprotection reaction product.