Adsorption layers of aqueous solutions of the amphiphile N-dodecyl-gamma-hydroxybutyric acid amide are investigated using Brewster angle microscopy (BAM) and surface pressure (pi) measurements. The experimental conditions for the occurrence of a first-order phase transition in adsorption layers (Gibbs monolayers) are studied. This phase transition is indicated by an inflection point in the pi-t adsorption kinetics and the evolution of the condensed phase structures whose the morphological features are visualized by BAM. Langmuir monolayers of this amphiphile can be obtained by the usual monolayer technique and are compared with the adsorption layers. The pi-A isotherms of the Langmuir monolayers show the plateau region that corresponds to the two-phase coexistence region for the main transition between a fluid phase of low density and a condensed phase. The critical surface pressures for the main phase transitions in the adsorption layers and Langmuir monolayers are correlated. The morphological features of the condensed phase structures of the Langmuir monolayers and adsorption layers agree completely.