Photocatalytic degradation in multiple contaminant system with a nitrogen-doped TiO2 visible-light photocatalyst was investigated. Formaldehyde and toluene were chosen as components of the contaminant. The degradation rate of toluene was depressed by coexisting formaldehyde, while that of formaldehyde was scarcely affected by coexisting toluene. This phenomenon is attributed to preferential adsorption of formaldehyde on the photocatalyst powder.