One of the major barriers in the development of applications using NiTi shape memory alloy films is the difficult control over their chemical composition. In fact, the transformation temperatures of NiTi are very sensitive to the alloy composition. In this paper we show that optical emission spectroscopy can be used to monitor the composition of NiTi films during sputter deposition. The parameter controlling the composition of the films is the product of sputtering gas pressure and target-to-substrate distance. The influence of this parameter is discussed in terms of thermalisation of the sputtered atoms. We find a linear relationship between the ratio of optical emission intensity of Ni and Ti atoms and the Ni concentration in the films.