Abstract: In the present study 27 different quinazoline derivatives have been synthesized and investigated as substrate or inhibitor for molybdenum hydroxylases. These compounds have been identified using NMR, mass spectrum, infrared and elementary analysis. In vitro, the oxidation of xanthine and phthalazine by xanthine oxidase and aldehyde oxidase from guinea pig liver, respectively, had been inhibited notably by 6-iodo-quinazolines. Although xanthine and phthalazine are excellent and specific substrates, allopurinol (100 μM) and menadione (100 μM) as specific inhibitors of xanthine oxidase and aldehyde oxidase, respectively, have been used to characterize the specificity of reaction. The inhibitory specificity as well as the active site requirements have been discussed and compared with their relative lipophilicities. 6-Iodo-substituted quinazolines inhibit both aldehyde oxidase and xanthine oxidase in a competitive pattern with Ki or IC50 ranging from 48 to 700 μM. This study indicates strongly that un-fused pyrimidine ring is required for inhibitory activity of quinazoline derivatives (see Q17 and Q21).