Aromatic azides are reasonably stable and can be easily synthesized without drastic reaction conditions. The most popular method was reported by P.A.S. Smith. Following Smiths's method, an azido derivative, namely: di-(p-azidophenyl)chlorophosphate (DAPP) has been prepared. The prescence of azido groups in the molecule enables its direct linkage with solid supports and proteins by a simple photolysis procedure. The newly synthesized compound has strong ultraviolet absorption at 276 nm and the characteristic azide band at 2100/cm. Unlike diisopropylflourophosphate (DFP), neither DAPP nor its irradiated product had any inhibitory effect on horse serum cholinesterase. However, simultaneous presence of DAPP and the enzyme in the reaction mixtures under UV irradiation resulted in strong inhibition of the enzyme activity. The light-independent DAPP inhibition of cholinesterase depended on DAPP concentrations present. The concentration that yielded half-maximal velocity was in the range of .0000001 M and the presence of .00001 M DAPP almost completely inhibited the enzyme reaction. The light-dependent DAPP inhibition of cholinesterase resulted in the reduction of both K subm and V sub max values of the enzyme system. Since the reciporcal plots of the kinetic data did not intersect at a common point, and also because the slopes vs concentrations replots were nonlinear, the presence of two or more nonmutually exclusive inhibitor binding sites is predicted.