A polyphosphoester (PPE) with phenyl phosphate groups separated by precisely 20 CH2 groups in its backbone was dissolved in chloroform and spread on the water surface of a Langmuir trough. The surface pressure vs area per monomer unit (π-A) Langmuir isotherm together with epifluorescence and Brewster angle microscopy suggested that some crystallization occurred already at zero surface pressure immediately after solvent evaporation of the spread polymer solution. The extended plateau region of the Langmuir isotherm in the range of π ≈ 4.5 mN m−1 corresponds to the 2D crystallization of most polymer chains. Brewster angle and epifluorescence microscopy show that during the crystallization of PPE in the Langmuir film single crystal like hexagonal entities are formed with lateral dimensions of up to 20 µm. These entities break upon compression beyond the limiting area per monomer unit which leads to a decrease of the elasticity modulus of the Langmuir film. The morphology of the single crystals and their failure upon compression are also observed in Langmuir-Blodgett films by atomic force microscopy. The thickness of the crystals is about 2.5 nm at the end of the plateau range and reaches 3.0 nm upon further compression. The polymer crystallization on the water surface is also confirmed by infrared reflection absorption spectroscopy.