Using both vibrational sum-frequency generation and fluorescence microscopy, the phase behaviour of a DPPC lipid monolayer on water is investigated as a function of surface pressure. The vibrational specificity of the sum-frequency generation techniques permits determining the order of the alkyl chains, as well as the average orientation of the terminal methyl group. A novel - and extremely sharp - phase transition is observed at low compression, which is attributed to a curling of the alkyl chains due to increased exposure of the (hydrophobic) alkyl chains to the water surface.