Conclusive evidence is presented for a first-order phase transition in adsorption layers. It is based on the study of special tailored amphiphiles, e.g. N-dodecyl-gamma-hydroxybutyric acid amide (DHBAA). The general conditions necessary for the formation of a two-phase coexistence in adsorption layers are investigated using surface pressure (pi) transients and Brewster angle microscopy (BAM). During the adsorption kinetics, appearance and location of the phase transition paint depends largely on the concentration of the amphiphile in the aqueous solution and on the temperature. Different types of morphological textures of the condensed phase can be formed above and below 10 degrees C. The experimental bridging to the Langmuir monolayers supports the conclusions of a first-order main transition during the adsorption kinetics. The experimental results are in agreement with a theoretical model which allows the description of the adsorption kinetics of the two-dimensional first-order phase transition in an adsorption layer.