The transport of electrons and ions to metal substrates in low-temperature plasma containing negative ions is studied by a computer experiment. The used method of simulation was the PIC-MC technique in simplified geometry. The modelled O-2/Ar plasma consists of positive argon and oxygen ions, negative oxygen ions and electrons in various proportions. In the simulation the main attention was devoted to the influence of negative ions on the sheath formation, both in steady state and dynamic regimes. (C) 2003 Elsevier B.V. All rights reserved.