The crystalline properties of the amphiphilic systems N-methylarachidamide (NM) and of N,N’-dimethyldocosanediamide (NND) in the uncompressed state at 5 degrees C on surfaces of water and liquid formamide were studied by grazing incidence synchrotron X-ray diffraction (GID). On water, the amphiphile NM forms a crystalline monolayer. On formamide, NM behaves differently. One h after deposition on the liquid surface, GID patterns indicated coexistence of two different crystalline monolayer structures. Three h later, a crystalline bilayer had formed. The fundamental question as to whether the interlayer growth took place in the liquid subphase or in the air was elucidated with the help of the tailor-made additive N-benzylarachidamide (NB). Addition of 10% of NE in the spreading solution yielded only a monolayer. From this result we conclude that bilayer growth of pure NM must occur from the liquid side. It also helps to establish a similar interlayer growth mechanism for the trilayer formation of pure arachidamide on liquid formamide.(1) The amphiphile NND behaved differently from NM as it formed a crystalline monolayer on both water and formamide liquid surfaces.