Based on the principle of electron transfer in the process of photocatalysis, this study uses a smectite clay catalyst with semiconducting properties to degrade the target pollutant 4-chlorophenol (4-CP). Multivariate analysis is adopted to address major factors affecting the degradation efficiency of smectite catalyst. After calcinations at about 400 degrees C, each type of smectite catalyst is observed by TEM and the mean transverse diameter of the synthesized catalyst is about 10-50 nm. The result of photocatalysis among the four types of smectite catalyst such as smectite-Ag, smectite-Zn, smectite-Ti and smectite-Ag/Zn on the degradation of 4-CP shows smectite-Ti catalyst to have the best effect. On the other hand, smectite-Ag/Zn catalyst works better than single smectite-Ag and smectite-Zn catalyst under the same condition. Due to competitiveness between intermediate products and targets for reactive sites of (OH)-O-center dot in the later stages of photocatalysis, there is a tendency of a decrease in the reaction rate of the smectite-Zn catalyst. Any amount of catalyst greater than 0.1 wt.% will, due to shielding effects, decrease the utilization of light energy and degradation efficiency. The higher the pH value, the more (OH)-O-center dot radicals will be generated to react with organic matter and increase reaction rate. (c) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.