The reaction of 2-aminobenzimidazole with ethyl cyanoacetate gave access to an efficient synthesis of 4-amino-1H-benzo[4,5]imidazo[1,2-a]pyrimidin-2-one (1) in excellent yield. A series of novel phenylazopyrimidone dyes were prepared by linking o-, m-, p-nitroaniline, o-, m-, p-chloroaniline, o-, m-, p-anisidine, o-, m-, p-toluidine and aniline to 4-amino-1H-benzo[4,5]imidazo[1,2-a]pyrimidin-2-one (1). The prepared compounds were characterized by UV-vis, FT-IR and H-1 NMR spectroscopic techniques and elemental analysis. The effect of varying pH and solvent upon the absorption ability of phenylazopyrimidones substituted with electron-withdrawing and electron-donating groups at their o-, m-, p-position was examined. (c) 2005 Elsevier Ltd. All rights reserved.