A novel process for H2S gas treatment has been introduced, based on the combined action of a chemical absorption step and a biological step involving the biocatalytic activity of the bacterium Thiobacillus ferrooxidans. The present authors also contributed some work on the subject, highlighting some features of the process, particularly on treatment of low concentration emissions, which are particularly difficult to deal with using ’conventional’ treatment processes. The aim of this paper is therefore to deepen the knowledge of the process for hydrogen sulphide elimination from gas streams based on that chemical/biological method. The development of an immobilized biomass reactor system is discussed, suitable for application of the process of H2S absorption with chemical reaction in a ferric solution and the consequent biological oxidation of ferrous iron fbr regeneration of the absorbent solution. A special interest is devoted to the study of the performance of the chemical stage and influence of this step on the overall process performance. A double stage plant was used for the experimental work, treating an H2S gas stream with concentrations as low as a hundred ppm. High efficiencies were observed and the type of possible chemical reaction absorption regimes have been identified.