Biogas hydrogen content varies with the operational conditions of an anaerobic digester, and may be a useful control parameter. Experiments were carried out on a laboratory scale fluidised bed anaerobic digester, fed with a synthetic baker's yeast wastewater. Step overloads produced a sharp peak in biogas hydrogen level measured on-line, e.g. an increase of loading rate from 40 to 63 kg COD m-3 day-1 increased hydrogen concentration from 290 to 640 ppm within 3 h. However, switching from an older, partly acidified batch to a fresh batch of feed at constant COD gave a marked increase in the biogas hydrogen content from 200 to 800 ppm. In these studies propionic acid and biogas hydrogen concentrations were not linearly correlated. According to the present results, biogas hydrogen content is unsuitable as a stand-alone control variable for anaerobic digestion, but its rapid response and ease of on-line measurement supports its use in digester control along with other liquid phase parameters to be measured on-line.