The dissociative excitation reaction of BrCN induced by the products of the electron cyclotron resonance (ECR) plasma flow of He was studied based on the electrostatic-probe measurements and on the optical emission spectra of the B 2 Σ + −X 2 Σ + transition of CN radicals. The partial pressures of He and BrCN were 3 and 1mTorr, respectively, and the partial pressure of H 2 O, PH2O, was in the range of 0.0–0.6mTorr. The electron density, n e , showed a negative dependence on PH2O as (2.63±0.13)×10 12 −(0.23±0.10)×10 12 m −3 , and the electron temperature, T e , a positive dependence, (2.38±0.36)−(4.51±0.15)eV. The CN(B 2 Σ + −X 2 Σ + ) emission intensity showed a negative dependence on PH2O. Based on a kinetic analysis of these PH2O dependencies, the decomposition of BrCN does not proceed via electron impact; instead, decomposition proceeds via the processes involving He + and/or He metastable atoms.