The WTi films were deposited by an unbalanced magnetron sputtering of a WTi (70:30 at. %) alloy target. The influence of the working gas (Ar) pressure, substrate bias, and substrate location on the composition of films was studied. The films deposited at low working gas pressures (<1 Pa) onto electrically floating substrates were largely depleted in Ti while the composition of films deposited at high argon pressure (25 Pa) was close to that of the target. The ion bombardment of the growing film resulted in a decrease of the Ti content in the films. The composition of the films deposited simultaneously onto a pair of substrates placed at the axis and at the periphery of the target did not depend on the substrate position at both low and high pressure. Further studies were carried out for a better understanding of the underlying processes affecting the film composition. Namely, the mass-resolved ion energy distribution, function at the substrate position was measured for various pressures. Further, the composition of the flux towards the target (backward flux) was studied as a function of pressure by Rutherford backscattering. Finally, the direct simulation Monte Carlo (DSMC) computer simulation of the gas-phase. transport of sputtered species was carried out. The results of the DSMC simulation (film composition, backward atomic flux, and ion energy distribution at the substrate) were compared with, the experimental results. The formation mechanism includes the simultaneous action of two competing factors. One factor is the resputtering by fast argon neutrals reflected from the cathode and/or by plasma ions accelerated by the substrate bias, resulting in films deficient in Ti. The other factor is the gas phase scattering on the background gas with a twofold effect. On one hand the scattering leads to the reduction of energy of fast neutrals and consequently to diminishing of the resputtering effect. On the other hand, under conditions of comparable values of the mean free path and the substrate-to-target distance the difference in scattering of various sputtered species can lead to the alteration of the composition of deposited films.