Arsenic (As) is a naturally-occurring group A carcinogen found in food, water and soils worldwide. There is a growing body of evidence pointing towards dissolved organic matter (DOM) as being a key player in the control of arsenic redox state, mobility and toxicity. Yet, no known study has directly quantified DOM effects on the fraction of arsenic available to bacteria. Because of its demonstrated binding affinity to As, we hypothesized that DOM serves as a strong predictor of As bioavailability. Using an As-specific biosensor, we tested our hypothesis by altering environmental variables affecting the affinity of As to DOM, namely DOM photoreactivity, As-DOM complex age, ionic strength and DOM origin selected for its sulphur content. First, in the presence of DOM, we demonstrated that As bioavailability was affected by DOM concentration, the age of the As-DOM complex and by the presence of divalent cations. We showed that DOM differentially affected the bioavailability of As(III) and As(V) and that bioavailable concentrations of both inorganic As species were reduced by up to 83% in the presence of environmentally relevant levels of calcium (10 mM), likely a result of cationic bridging (As-Ca-DOM). Here, DOM aromaticity, sulphur and carboxyl content appeared to control the extent of As species uptake. Second, the combination of the biosensor and ICP-MS measurements revealed that As(III) was readily photooxidized regardless of the presence of DOM, and that changes in ionic content greatly affected the extent of As(III) photooxidation in the presence of DOM. Our results are the first to reveal the control that DOM exerts on As(III) and As(V) bioavailability to bacteria. We emphasized the complex role that DOM plays in arsenic photoreactivity and its consequences on As bioavailability. Our work is relevant to understand the biogeochemistry of arsenic in the context of on-going environmental changes such as prolonged seasonal irradiation, seasonal redox fluctuations and As cycling in ground/surface waters with high organic matter content and high ionic strength.