Arsenic in drinking water and soils poses a serious threat to millions of people worldwide due to its chronic toxicity. The oxidized arsenate (As(V)) is less mobile and toxic than the reduced arsenite (As(III)). The reduction of As(V) to As(III) in most environmental systems is driven by microorganisms, but arsenic sorption at the mineral-water interface may strongly influence the rates of this transformation. Understanding the influence of mineral surfaces on the kinetics of microbial As(V) reduction is thus essential to better assess the arsenic mobility and toxicity in soils, sediments, and natural waters. In this project, incubation experiments will be performed to study the influence of As(V) sorption on Fe and Al (hydr)oxide surfaces on the kinetics of the microbial As(V) reduction. The effects of microbe-mineral surface interaction, microbial activities and competition between As(V) and Fe(III) reduction will be studied in detail. The experiments are carried out with arsenic reducing bacteria to focus on the effect of arsenic sorption on As(V) reduction and with iron and arsenic reducing bacteria to study the competition between As(V) and Fe(III) reduction. The biogeochemical fate of arsenic at the water-solid interface is considered parallel from the aspects of microbial activities, chemical speciation and physiochemical processes. The knowledge generated in this study provides new insight into an important environmental process and is needed for the implementation of powerful strategies to cope with the problem of large-scale arsenic release from sediments into water worldwide.