External infrared reflection spectroscopy was applied to study a phase transition of the monolayer of octadecylurea (OU) at the air-water interface. Upon compression at a constant; temperature (6 degrees C in this study) the monolayer undergoes the transition from a condensed phase (beta phase) with a larger limiting area (0.250 nm(2)/molecule) to another condensed phase (alpha phase) with a smaller limiting area (0.195 nm(2)/ molecule). The formation of the a phase and the phase transition accompany discrete IR spectral changes in the V-as(CH2), v(s)(CH2), amide I, amide II, and delta(NH2) vibration regions. The spectral changes indicated that (i) upon compression of the monolayer to form the beta phase the alkyl side chain is converted from an irregular state containing gauche conformations to an ordered one consisting of the all-trans conformation and at the same time the peptide bond in the head group is transformed from a fully hydrated, state to a hydrogen-bonded one and (ii) upon further compression to the alpha phase and/or a collapsed slate the alkyl side chains are changed again to the irregular state and a part of the peptide bonds is converted to the hydrated state. These results are contrasted with the case of the monolayer of N-methyl-N-octadecylurea (MOU) at the air-water interface, which gives the amide I and delta(NH2) bands characteristic of a fully hydrated state irrespective of the molecular area. Thus, the IR spectral changes provided direct; proofs for the explanation that the endothermic fiat-order beta -> alpha phase transition of the OU monolayer takes place through a partial collapse of the two-dimensional ordered array consisting of the hydrogen-bonded network between the peptide bonds.