Phase transitions, morphology and crystal structure of monolayers at the air-water interface of N-alkyl-gamma-hydroxy-butyric acid amides with different alkyl chain lengths have been studied by pi-A isotherms, Brewster angle microscopy (BAM) and synchrotron grazing incidence X-ray diffraction (GIXD). While the morphological structures were similar, the pi-A isotherms and the oblique crystal structure indicate a marked difference between the molecular area in the condensed phase of acid amides with shorter (DHBAA, THBAA and PHBAA) and longer (HHBAA and OHBAA) alkyl chains, resulting from a change in the conformation of the alkyl chains. The increase in molecular area with increasing van der Waals interaction (increase of chain length) can be explained as a result of the competition between the van der Waals interactions of alkyl chains and the rigid hydrogen bondings of acid amide groups.