The global bacterial AsIII-oxidizing activity tends to decrease As toxicity in soils and its transfer toward underlying aquifers. The influence of different types of organic matters on arsenic speciation by pure strains and soils microflora was determined. Experimental results show that the presence of organic matter has a negative effect on the specific rate of AsIII oxidation by pure strains presenting different metabolisms. In parallel, experiments are performed with various soils from different types of polluted sites: former mining and industrial sites, forest and agricultural soils impacted by mining activities. The effect of yeast extract on the AsIII-oxidizing activity of the soil microflora is evaluated while considering the potential influence of soil parameters. Results suggest a variable influence of yeast extract on AsIII oxidation kinetics according to the range of yeast extract concentration. As a whole, experimental data strongly suggest a negative effect of organic matter on bacterial AsIII oxidation.