The release of arsenic (As) from As-rich riverbed sediments (Anllóns River, Spain) was studied. In this area As is associated with pyrite and arsenopyrite and its mobilization has been favoured by past gold-mining activities. The effect of pH and phosphorous (P) concentrations, as well as the kinetics of the As release process were studied. Also, As species in solution were determined as a function of pH and time. The release of As was greatly influenced by pH and it was between 10 and 45 times higher at pHstat 10 than at pHstat 4 for all the sediments studied. The percentages of As released were between 0.1 and 0.8 % of total As at pHstat 4 but increased to values between ~7 and 11 % of total As at pHstat 10. The sediment containing the highest concentration of As released the highest amount of As (~18 mg/kg at pHstat 10). The As release kinetics was also greatly influenced by pH, mostly at alkaline pH. At pHstat 10, the amount of released As increased progressively with time. In contrast, at pHstat 4 only small differences were observed after 24 h in the amounts of As released. At alkaline pH As was released along with Fe and Al, and accompanied by organic matter dissolution. This suggests a determining role of organic matter in the As release behaviour in alkaline media. In contrast, at weakly acid pH, the release of these components was very low, suggesting that As release is controlled mainly by desorption. The speciation of As was influenced by the pH and the release time. At pHstat 4 the major species in solution were As(III) and As(V) (representing 45–69 % and 51–29 % of As released, respectively). In contrast, at pHstat 10 almost all the released As was As(V) (~99 %). The amount of As released from sediments increased with the increase in initial P concentration in solution and it experienced a dramatic increase at added P concentrations above 1 mM. Namely, the percentage of As released by 10 mM P reached ~6 % of total As for the sediments richer in As. The results obtained let us conclude that changes in pH, especially alkalinization, and inputs of P to the river markedly enhanced As mobilization from the Anllóns River sediments, thus increasing health risks.