The (S)-4-alkoxo-2-azetidinecarboxylic acids are optically active beta-lactam derivatives of aspartic acid, which are used as precursors of carbapenem-type antibiotics and poly-beta-aspartates. The crystal structures of three (S)-4-alkoxo-2-azetidinecarboxylic acids with alkyl chains with 10, 12 and 16 C atoms were solved using parallel tempering and refined against the X-ray powder diffraction data using the Rietveld method. The azetidinone rings in the three compounds display a pattern of asymmetrical bond distances and an almost planar conformation; these characteristics are compared with periodic solid-state, gas-phase density-functional theory (DFT) calculations and MOGUL average bond distances and angles from the CSD. The compounds pack along [001] as corrugated sheets separated by approximately 4.40 A and connected by hydrogen bonds of the type N-H...O.