In this study poly(beta-hydroxybutyrate) (PHB) was used as a spreading material to form LB monolayers at the air-water interface. Isotherms were found to show a transition at about 14 mN/m that is argued to be associated with a phase transition in the monolayer. Stable monolayers could be obtained at various surface pressures before as well as beyond the transition. Multilayers on substrates were investigated with FT-infrared techniques and were shown to have crystalline characteristics. Hysteresis experiments confirm the occurrence of irreversible processes in the monolayer during compression. Transmission electron microscopy pictures clearly show the structural changes that appear with increasing stabilization surface pressure of the monolayer. At large areas PHB exhibits an expanded monolayer behavior. Under the influence of surface pressure PHB is argued to change into a crystalline structure and eventually to form a bilayer of helical molecules which is reflected in the shape of the isotherm. S-shaped stabilization curves are argued to be the result of an accelerated bilayer formation process, as is confirmed by TEM pictures of this layer. Infrared external reflection spectroscopy of a PHB monolayer on the water surface gives clear indications that PHB already crystallizes at the air-water interface during compression, thus confirming our theorem.