Abstract: A pseudo first order Langamuir-Hinswood expression is generally used to describe heterogeneous photocatalytic kinetics, but it still uncertain whether the oxidation process occurs on TiO2 surface or in solution. Adsorption behavior and photocatalytic oxidation of methyl orange and naphthalene scarlet 3R on P-25 TiO2 have been studied in order to understand the relationship of adsorption and reaction kinetics of the azo dyes. The results showed that the adsorption of both the azo dyes is obviously influenced by pH value in solution. Significant adsorption of both the azo dyes on the catalyst has been detected at pH=3,while the adsorption decreased obviously at pH≈6.The adsorption amount of naphthalene scarlet 3R is larger than that of methyl orange under cases of pH=3 and pH≈6 due to the structural difference between the two azo dyes. No adsorption was observed at pH=9 for both the azo dyes. Rates of photocatalytic degradation of the azo dyes in different media were in the order: the acidic > the basic > the neutral condition. These facts indicated that the photocatalytic reaction carried out under different mechanism in different media. It is proposed that the photocatalysis occurs on catalyst surface under acidic condition, which may follow L-H mechanism, whereas the photocatalysis occurs in solution under basic condition. A kinetic expression for the photocatalysis under basic condition was proposed, which can be finally simplified to an apparent first order reaction kinetic equation, similar to the pseudo first order L-H expression. This may be one of the reasons that the photocatalytic degradation kinetics usually can be described by the pseudo first order L-H expression.