Two hetarylazo indole dyes were analyzed by single crystal X-ray diffraction. Dye 1, C18H14N4O, crystallises in the monoclinic system, space group P21/c, with two independent molecules in the asymmetric unit. Dye 2, C20H16N4, also crystallises in the monoclinic system, space group P21/n, Z = 4. In both compounds, there are two intramolecular C-H...N hydrogen bonds which influenced the molecular conformation between the azo group and the indole ring system and its phenyl substituent. Each of the independent molecules of dye 1 interact through the N-H...N hydrogen bond, whereas no classical intermolecular hydrogen bond was observed in dye 2. The molecules of dye 1 and dye 2 are packed differently in both structures. © 2007 The Authors. Journal compilation 2007 Society of Dyers and Colourists.