In order to safely use biogas in applications which are highly sulfur-sensitive, such as fuel cells or catalytic methanation processes, methods for accurate online measurement of total sulfur at sub-ppm_v concentrations must be developed and validated. A method based on a sulfur chemiluminescence detector (SCD) is presented here for this purpose. Biogas-specific variations in gas composition were investigated for their effect on the technique. While the technique was neither affected by the use of different sulfur compounds nor by the existence of aromatic compounds (terpenes) in the biogas, it was found to be sensitive to the variable CH₄ content in the bulk gas as well as to variable gas humidity levels. These effects were reduced or eliminated by optimizing the combustion parameters in the SCD for the variable CH₄ effect and by using sulfur-inert materials to pretreat the biogas for removal of moisture. The effect of siloxanes on SCD measurements was discussed. After optimization of the operational parameters, a detection limit of 0.02 ppm_v (20 ppb_v) was achieved with no sample preconcentration in a gas matrix containing 60 v % CH₄ and 40 v % CO₂. Finally, the method was validated by demonstrating it in field operation on a slipstream of biogas produced from an industrial anaerobic digester processing biowaste.