Silver tungstate as photocatalyst for water splitting and dye degradation has been reported, but the catalyst stability is not known. In this work, we find that both alpha- and beta-Ag2WO4 are not stable under UV light for the photocatalytic degradation of phenol and azo-dye X3B in aqueous solutions. Comparatively, beta-Ag2WO4 was more photoactive, but less stable than alpha-Ag2WO4. Solid characterization with X-ray diffraction and scanning electron microscope showed that metallic silver particles were produced with the two catalysts, consequently resulting into decrease in the activity for organic degradation. Measurement of photoluminescence revealed that beta-Ag2WO4 had a weaker band gap emission and higher portion of structural defects than alpha-Ag2WO4. A possible mechanism responsible for the observed difference in photoactivity and stability between the two tungstates is proposed. (C) 2014 Elsevier B.V. All rights reserved.