In this paper, we found that the acidic and basic dyes were easily decolorized by a bis-ions coexistence system of NH4+ and NO3- under UV light irradiation. The coexistence of NH4+ and NO3- is a necessary condition for the photocatalytic decolorization of soluble dyes. The photocatalytic decolorization of methyl orange (MO) and methylene blue (MB) follows the first order rate kinetics. The location of an absorption peak in the visible region is blue-shifted with the increase in the illumination time. It is proposed that the photocatalytic decolorization of soluble dyes in the bis-ions coexistence system of NH4+ and NO3- is a photoreduction reaction, in which the ammonium nitrate acts as a photocatalyst. The chromophore of acidic and basic dyes reacts with hydrogen and then results in their rapid decolorization. (C) 2010 Elsevier B.V. All rights reserved.