The synthesis of the p-nitrophenyl esters of the 5′- and 3-phosphates of the nucleoside analogue 2′,3′-secouridine are described. Unlike the corresponding diesters of thyinidine, these two compounds are diastereoisomers. Their affinity for phosphodiesterases types I and II were investigated. Both analogues were hydrolysed very slowly by snake venom phosphodiesterase but their affinity for the enzyme was similar to that of the p-nitrophenyl ester of thyrnidine 5′-monophosphate of which they were both competitive inhibitors with Kjxs Km. Neither compound was hydrolysed by spleen phosphodlesterase but both competitively inhibited the p-nitrophenyl ester of thymldine 3′- monophosphate, with Kj's slightly higher than the Km. Although for each enzyme the Kjof the correct′analogue phosphodiester (i.e. the 5′-derivative for snake venom and the 3′for spleen) was the lower, the absolute specificity seen for the normal substrate