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. N2 ∼ 54-67 vol.%; H2 ∼ 5-13 vol.%; CO2 ∼ 15-17 vol.%; CO ∼ 10-17 vol.%), and lower levels of CH4 (∼1-2 vol.%) and O (∼0.1-1 vol.%) in the gas. Trace levels of HS (∼100-300 ppmv) and COS (∼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 HS varied from 200 to 700 ppmv, and COS from 7 to 17 ppmv. The ratio of HS: COS was higher than in the laboratory trials - probably because of COS hydrolysis reactions taking place in the wet scrubbing systems.