Biological processes have proved to be suitable for desulphurization of energy-rich gases containing high loads of H2S(g). However, monitoring devices for assessing such bioreactors performance are still scarce. In this work, an analytical system consisting in a flow injection analyser (FIA) for total dissolved sulphide (H2S(ag) + HS- + S2-) (TDS) and a continuous flow analyser with a gas diffusion step (GD-CFA) configuration for H2S(g) have been used for on-line monitoring of a biotrickling filter for desulphurization of biogas mimics under aerobic conditions. Analyses were validated versus commercial sensors during real-time monitoring of the biotrickling filter performance under aerobic and anoxic conditions. During the aerobic period monitoring reported herein, the inlet H2S concentration was stepwise increased from 2000 ppm(v) (51 g H2S m(-3) h(-1)) up to 8000 pPm(v) of H2S (215 g H2S m(-3) h(-1)). Complementary off-line measurements of other sulphur species allowed assessing a sulphur mass balance, which indicated that S-0 production proportionally increased with an O-2/H2S ratio decrease. Under the studied conditions, the maximum TDS concentration detected in the liquid phase was 3.08 mg S2- L-1 while the maximum elimination capacity of the reactor was 201 g H2S m(-3) h(-1) and the maximum removal efficiency was 100%. The FIA/GD-CFA results demonstrated that the proposed analytical system is reliable for on-line monitoring applications and can serve as a valuable tool to understand the complex mechanism of biological desulphurization. (C) 2010 Elsevier B.V. All rights reserved.