Ethylene is unstable above certain limits of pressure and temperature decomposing into carbon, hydrogen, and methane. The influence of comonomers on this limit were studied. Vinyl acetate, acrylic acid, methyl methacrylate, I-butene, and 1-hexene were added in various concentrations to ethylene and by filling the mixture into a preheated autoclave under high pressure (up to 450 degrees C and 150 MPa). As a function of starting pressure and temperature, two reaction pathways can be distinguished: (I) stable (co-) polymerization and (2) (co-)polymerization with decomposition. Lowering the boundary between these two pathways seemed to be caused mainly by the thermal stability of the comonomer, for mixtures with vinyl acetate and methyl methacrylate, while an inhibition effect and increase in the limits occur in the comonomer acrylic acid, 1-butene, and 1-hexene. The maximum values of pressure, pressure ina ease, and temperature during decomposition were also influenced by comonomers. The maximum explosion pressure as a function of mixture density has a maximum for pure ethylene. As a function of ethylene concentration, the maximum explosion pressure has minimum values for pure ethylene. At a constant wall temperature and small initial pressures, the maximum rate of pressure increases reached a maximum, while, at high pressures, it decreased again. The position of this maximum depends on wall temperature and comonomer. Adding comonomers to ethylene decreases the maximum temperature es during decomposition reaction.