Polydiacetylene (PDA) Langmuir films (LFs) were investigated directly at the air/water interface using in situ synchrotron grazing incidence X-ray diffraction, and ex situ transmissison electron microscopy and diffraction. The films were compressed and polymerized on pure water. A crystallographic model describes the structures and phase transitions of the unpolymerized (monomer) film, via the metastable (blue phase), to the fully stable PDA red phase as a function of irradiation dose. The monomer-to-blue-to-red chromatic phase transitions are accompanied by changes in the in-plane crystal structure and pendant chains packing arrangement from arced alkyl chains (in the monomer and blue phases) to near-vertical closely packed chains in the red phase. Notably, the characteristic linear strand morphology of PDA films can be explained as a direct result of the marked decrease in spacing between adjacent polymer chains upon transition from the blue to the red phase.