The acid-catalyzed degradation mechanism of chemical amplification resists based on end-capped poly(phthalaldehyde) has been elucidated by semi-empirical molecular orbital calculations. It is concluded that : (i) two different starting points exist in the degradation of end-capped poly(phthalaldehyde), the central part of the polymer as well as the polymer end; (ii) in both cases, after the first protonation, cationic hydroxyl compounds generated decompose to the identical cation intermediate, which can unzip itself to phthalaldehyde monomers successively (unzipping reaction); (iii) the hemiacetal type intermediates hardly degrade to the same intermediate without acid catalyst; (iv) the overall activation energy for the degradation reaction is less than 14 kcal/mol in all the cases. Thus, the poly(phthalaldehyde) resist can easily self-develop below 100 degrees C, as reported in the literature.