A new synthesis of 4-alkyl-1,3-oxazoline-4-carboxylic acids involves the thermal rearrangement of 4-alkyl-4-hydroxymethyl-1,3-oxazolin-5-ones prepared from N-acyl-2-alkylserines or N-protected peptides with a C-terminal 2-alkylserine residue. The rearrangement is fast and affords HPLC pure title compounds in 68-96% yield both from racemic and enantiomerically pure N-acyl-2-alkyl serines as well as from respective peptides.