Dolomite is a cheap and robust catalyst used for biomass gasification, but its deactivation under relevant conditions of pilot-scale gasifiers has still been poorly understood. For this reason, the catalytic activity of fresh and used dolomites produced from an industrial air-blow fluidized bed was investigated. Fresh and used dolomites were characterized by BET, SEM-EDX, XPS, ICP-MS, XRD, TPD and TPO. Benzene steam reforming was selected as a surrogate reaction of tar conversion in order to probe the reactivity of the two dolomites. The activity of used dolomite was 25% lower than that of fresh dolomite. This difference could be explained by: (1) the deposition of a Si-based layer from biomass ashes at the surface of used dolomite, and (2) the production of coke during gasification. The reaction mechanism of benzene steam reforming over fresh and used dolomites was discussed. For used dolomite, the Si and coke depositions reduced the availability of the active sites (CaO, MgO) thus lowering the conversion of benzene. These deposits could also inhibit the interactions between CaO and MgO and enhance the formation of a stable coke.