An efficient synthesis of 1-arylisochromeno[3,4- d ][1,2,3]triazol-5(1 H )-ones, involving the diazotization of 3-amino-4-arylamino-1 H -isochromen-1-ones in weakly acidic solution, has been developed and the spectroscopic characterization and crystal structures of four examples are reported. The molecules of 1-phenylisochromeno[3,4- d ][1,2,3]triazol-5(1 H )-one, C 15 H 9 N 3 O 2 , (I), are linked into sheets by a combination of C—H...N and C—H...O hydrogen bonds, while the structures of 1-(2-methylphenyl)isochromeno[3,4- d ][1,2,3]triazol-5(1 H )-one, C 16 H 11 N 3 O 2 , (II), and 1-(3-chlorophenyl)isochromeno[3,4- d ][1,2,3]triazol-5(1 H )-one, C 15 H 8 ClN 3 O 2 , (III), each contain just one hydrogen bond which links the molecules into simple chains, which are further linked into sheets by π-stacking interactions in (II) but not in (III). In the structure of 1-(4-chlorophenyl)isochromeno[3,4- d ][1,2,3]triazol-5(1 H )-one, (IV), isomeric with (III), a combination of C—H...O and C—H...π(arene) hydrogen bonds links the molecules into sheets. When compound (II) was exposed to a strong acid in methanol, quantitative conversion occurred to give the ring-opened transesterification product methyl 2-[4-hydroxy-1-(2-methylphenyl)-1 H -1,2,3-triazol-5-yl]benzoate, C 17 H 15 N 3 O 3 , (V), where the molecules are linked by paired O—H...O hydrogen bonds to form centrosymmetric dimers.