Boron nitride films were deposited by r.f. capacitively-coupled plasma (13.56 MHz) using two different gas mixtures: Ar-B2H6-NH3 and Ar-B2H6-N-2. In order to study the gas-phase reactions of the plasma in situ Fourier transform infrared absorption spectroscopy has been used. The study reveals that, in our conditions, mixing ammonia with diborane leads to the well-known creation of diammoniate of diborane. In the presence of the plasma, salt production is inhibited and creation of aminoborane is observed. Replacing ammonia by nitrogen, none of these species could be detected. The mechanism of formation of these products from the precursors and their influence on the deposited film are also discussed.