We present the results of an investigation into correlation between the type of film-forming species (FFS) (atoms and molecules) and the structure of the films deposited by reactive magnetron sputtering of W-30 at.%Ti alloy in an Ar-N-2 working gas. Mass-spectrometric studies have revealed that the FFS flux contains both atomic (W, Ti, N) and molecular (WN, TiN) species and the flux composition depends on the nitrogen partial pressure in a working gas. At low nitrogen partial pressures (P-N2<0.1 Pa) the FFS flux is mainly composed of W, Ti atoms and WN, TiN molecules and the film deposited from such flux is formed of biphase [W + (W,Ti)(2) N] composite material with extremely small (approx. 10 nm) grain size. The films deposited from the flux containing mostly metal and nitrogen atoms (higher nitrogen partial pressures P-N2 > 0.2 Pa) are formed from monophase (W,Ti)(2) N material with 2-4 times larger crystallite size. The microhardness of the films deposited from the FFS flux containing mainly metal atoms and nitride molecules was noticeably higher than that of the films formed mostly from metal and nitrogen atoms. (C) 2003 Elsevier B.V. All rights reserved.