Enzymatic oxidation of gaseous ethanol into acetaldehyde was conducted by using alcohol. oxidase in a packed bed reactor. Effects of the concentration of ethanol in a feed gas mixture, the water content in immobilized carrier particles, the flow rate of the feed, and the reaction temperature on the oxidation were investigated experimentally and analytically. The concentration of acetaldehyde at the outlet of the reactor increased with increasing reaction temperature and with decreasing water content, w(h), in the carrier particles and decreased with increasing gas flow rate. The concentration of ethanol in the feed gas mixture, however, slightly affected the reaction rate. It is also found that the yield concentration decreases with increasing process time, owing to an inhibition of the enzyme activity. These experimental results were discussed on the basis of a model analysis, which consists of mass balances of substrate and product in the gas and solid phases based on mass transfers and on a kinetic model of the reaction including the inhibition effect. The parameters used in the model were the volumetric mass-transfer coefficients and equilibrium constants of the substrate and product, the reaction rate constant, and the inactivation reaction constant. These parameters were evaluated experimentally. By the model, the experimental characteristics of the gaseous oxidation with an immobilized packed bed were well represented systematically.