Abstract <p>The article is devoted to the experimental and theoretical study of regular and complex oscillations during ethylene oxidation on the nickel foil. The simplest mathematical model was based on the 14-stage mechanism of reaction including the stages of oxidation and reduction of the Ni catalyst. A precursor-mediated adsorption of CO and C₂H₄ was shown to be the crucial condition for the origin of the oscillatory behavior under reducing conditions. It was demonstrated that for real values of the parameters, the mathematical model can simulate both regular and irregular oscillations, as well as the “mixed-mode” oscillations observed in the experiment. For the first time oscillations with different properties and distinct mechanisms of their occurrence were detected in the same model. It was demonstrated that oscillations occurred as a result of a strong dependence of the reaction rate on the concentration of active sites due to a variation in the concentration of the surface oxide or the surface carbon.</p>