It was confirmed by H-1 NMR measurements that poly(4-hydroxystyrene sulfone) (PHOSS) was decomposed in dissolving in a deuterium oxide solution of sodium deuteroxlde to give trans-2-(4-hydroxyphenyl)ethenesulfinic acid (HESA) and 4-hydroxystyrene (HOSt) quantitatively. HESA and HOSt were formed in the mole ratio of about 2:3. The amount of each product was almost independent of the alkaline concentration ranging from 1 to 12 w/v %. The mole ratio of monomeric units of decomposed PHOSS to NaOD was estimated to be 0.64. A possible decomposition mechanism was discussed. The initiation reaction was the abstraction of a methylene proton anti to a sulfonyl group by alkali, followed by the main chain scission of the polymer. The resonance structure stabilizing a terminal phenolate anion possibly enhanced the elimination of MESA, so that it was produced in quantity comparable to HOSt. The decomposition of PHOSS in alkaline aqueous solutions was a conformation-determined polymer degradation following Hofnnann's rule.