The equilibrium spreading behaviour of dimyristoyl phophatidylcholine has been studied at the air-water interface. The lipid was examined both as a dispersion (prepared by vortexing) and as the bulk solid (dropped onto the surface of water). The physical properties of the lipid in the aqueous media were examined by optical microscopy and cryo transmission electron microscopy. It was found that the nature of the sample was a key factor in the formation of a monolayer, spreading being dependent on the hydration of the bulk lipid and the formation of bilayers in the subphase. A model of bilayer formation is used to relate discrepancies in previously reported values of the equilibrium spreading pressure to a shift in the chain melting transition temperature T(m).