We have determined the initial cleavage rate of the cyclobutane ring of poly [4,4'-oxydiphenylene- 1,2,3,4-cyclobutanetetracarboximide] (CBDA-ODA) for linearly polarized ultraviolet light (LPUVL). The initial cleavage rates beta (//) and beta (perpendicular to) for LPUVL polarized parallel and perpendicular to the polyimide backbone direction, respectively were determined from the IR absorption data reported previously. beta (//) and beta (perpendicular to) are 0.70 +/-6.02 and 0.13 +/- 0.02 J(-1).cm(2), respectively, for LPUVL of wavelength 248 nm. We found that the anisotropy of the cleavage rate is much larger than that of the phenyl ring decomposition rate for poly[4,4'-oxydiphenylene-pyromellitimide] (PMDA-ODA) that was previously reported. The large anisotropy of the cyclobutane ring cleavage is advantageous to photo-induced alignment of liquid crystals based on the decomposition reaction of polyimide molecules.