A quantitative method of on-line gas analysis was developed, using quadrupole mass spectrometry to measure gas composition when charcoal, wood, straw, and refuse derived fuel were gasified. Despite overlapping between some of the species, the method was demonstrated on small-scale laboratory gasifiers, recording the main components (e.g. N-2 similar to 54-67 vol.%; H-2 similar to 5-13 vol.%; CO2 similar to 15-17 vol.%; CO similar to 10-17 vol.%), and lower levels of CH4 (similar to 1-2 vol.%) and O-2 (similar to 0.1-1 vol.%) in the gas. Trace levels of H2S (similar to 100-300 ppmv) and COS (similar to 10-30 ppmv) were also measured (important for gas clean-up strategies). On-line measurements were performed on a commercial pilot-scale down-draft gasifier (using waste wood), and the concentrations of H2S varied from 200 to 700 ppmv, and COS from 7 to 17 ppmv. The ratio of H2S: COS was higher than in the laboratory trials - probably because of COS hydrolysis reactions taking place in the wet scrubbing systems. (C) 2014 Elsevier Ltd. All rights reserved.