The photocatalytic activities of the isostructural photocatalysts MWO4 (M = Ca, Sr, Ba) for decomposing methyl orange, which were synthesized by a solid state reaction, were investigated. In the experiments, the photocatalytic activity is in the increasing order of CaWO4 < SrWO4 < BaWO4 under both neutral and acidic conditions. The factors, which influence the photocatalytic processes and the final activity, were analyzed the difference of their photocatalytic behaviors. The further investigation indicates that the higher structural openness degree, corresponding to a lower packing factor, leads to the better photocatalytic activity. (C) 2008 Elsevier B.V. All rights reserved.