Abstract The analysis of the high resolution FT-IR spectra of the perpendicular bands v7 (E') at about 1550 cm -1 and V9 (E') at about 1170 cm -1 of the isotopomers 12C314N3H3 , 13C3 14N3H3 , and l2C3I5N3H3 is given. Both bands proved to be free from accidental resonances. The molecular constants of the state v7 = 1 and v9 = 1 of the isotopomers under consideration are listed. The weak difference band v9-vl4 (E" → E') of 12C314N3H3 was recorded and analyzed, using the molecular constants of v9 = 1 [this work] and V14 = 1 [of 1995], This analysis proves the quality of the molecular constants of the fundamental v14 which is IR-inactive.