We have investigated the anisotropic decomposition of polyimide (poly[bis(4,4′-oxydiphenylene)-dimetylmetyl-pyromellitimide]) molecules induced by exposure to linearly polarized ultraviolet light (LPUVL) of wavelength ∼ 300 nm. The decomposition of the polyimide molecule was monitored by measuring the polarized infrared (IR) absorption spectra as a function of LPUVL exposure. We propose an empirical equation that describes the relation between the orientational distribution of polyimide chains and LPUVL exposure. We consider two decomposition rates β/ and β⊥ for the polyimide chains oriented parallel and perpendicular to the polarization direction of LPUVL, respectively. By taking account of the increase of the decomposition rates around 30 J·cm−2, the IR absorption data could be reproduced with β/β⊥ = 1.23 ± 0.02. The decomposition rate β/ is (4.5 ± 1.0) × 10−3J−1·cm2for the LPUVL exposure range up to -30 J·cm−2, and (1.6 ± 0.1) × 10−2J−1·cm2beyond -30 J·cm−2.