Lignite gasification may be characterized by the active site restriction result from deactivation. As was aimed here, a deactivation model (DM) can be developed by "pseudo-steady-state" mass balance, i.e. describing variation of available surface with time. For this purpose, bench-scale fluidized bed reactor (FBR) was operated for obtaining gasification product profiles. Gas samples were taken from the reactor effluent stream, and fed continuously to the gas analyzers for on-line simultaneous measurements of H-2, CO2, CO, CH4 and O-2. Data were used for extracting model parameters (k(//) and k(D)) from the linear form of DM. Experiments at temperature range of 973-1173 K were conducted. Fluidizing mixture (air and steam) entered the bed through a distributor of a 200-mesh stainless steel sieve and fluidized the single charge of lignite with a mean particle diameter approximately 250 mu m. The agreement between the experimental and predicted carbon concentrations was confirmed for DM. The latter may be successfully used to design the fluidized bed combustors or gasifiers.