Low-temperature ESR studies of copper pyrimidine dinitrate, a spin- antiferromagnetic chain with alternating g-tensor and the Dzyaloshinskii–Moriya interaction, are presented. The ESR linewidth and g-factor shift are investigated as function of temperature. The data are interpreted in frame of a new theoretical concept proposed recently by Oshikawa and Affleck [Phys. Rev. Lett. 82 (1999) 5136] and applied for precise calculations of the ESR parameters in antiferromagnetic chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained.