The effect of humidity on decomposition of ethylene by a spiral-wound wire-net photocatalyst was investigated to obtain kinetic information for the design of an efficient photocatalytic reactor. Photocatalytic decomposition of ethylene by titanium dioxide was found to proceed more rapidly the tower the humidity. Kinetic analysis was performed on the assumption that ethylene and water vapor molecules were adsorbed competitively on the same active site, and photocatalytic decomposition was controlled by the surface reaction step of adsorbed ethylene. At water vapor concentrations from 10,200 to 28,300 ppm, and the ethylene concentrations from 30 to 100 ppm, the reaction rate equation is expressed as follows: r=3.70 (.) 3.18x10(-2)C(C2H4)/1+3.18x10(-2)C(c2h4)+4.95x10(-5)C(H2O) where C-C2H4 [ppm] is the concentration of ethylene, C-H2O [ppm] is the concentration of water vapor, and r [ppm (.) min(-1)] is the decomposition rate of ethylene. The decomposition rates evaluated from experimental data are in fair agreement with the calculated ones.