A stochastic model representing the state of the catalyst surface is developed for several possible reaction mechanisms. With the pseudo-steady state assumption, the fresh catalyst solution was used for the initial value of deactivating system. The deactivation functions obtained from the present study shows a good agreement with the mod. 1 proposed by Nam and Froment[1]. When the reaction requires more than one site, the reaction becomes structure sensitive. However, for a single site reaction requiring one site only, the reaction belongs to the class of structure-insensitive reaction. Deactivation reactions can also be classified by this manner.