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The evolution of the C(1s) x‐ray photoelectron spectrum is examined as a function of the size of the polynuclear aromatic molecules. The transition from a symmetric line shape to an asymmetric one occurs in the narrow range of the layer size between 10 and 22 Å. It is shown that the asymmetry reflects the area of the layer over which the π electrons can be delocalized and that it is more closely related to the actual physical size of the layer than measurements from x‐ray diffraction. The latter is illustrated in coal tars which have undergone heat treatment. The effect of the evolution of the line‐shape asymmetry on the determination of the fraction of polynuclear aromatic carbon in heterogeneous carbon materials by x‐ray photoelectron spectroscopy is also discussed.