For the safe and trouble-free operation of a manufacturing plant and the safe storage of acrylic, as well as methacrylic monomers. it is important to know the polymerization stability as a function of the process parameters (temperature. oxygen concentration, and impurities. e.g., metal ions). Contamination with metal ions can be caused by the corrosion of steel units. Therefore. the influence of the metal ions Cr3+, Fe3+, Ni2- and Cu2- in the concentration range of 0-10 ppm (g g(-1)) on the polymerization behavior and the oxygen consumption of acrylic and methacrylic acid were examined in this work. It was shown that Cr3+. Ni2+, and Cu2+ ions extend the inhibition period of acrylic acid (AA) and methacrylic acid (MAA) and reduce the O-2 consumption. Fe3+ ions. however. cause a decrease of the inhibition period and in the case of AA an increase of the O-2 consumption. which leads. in the end. to a faster unintentional polymerization. Therefore. alloys which contain iron should be avoided as far as possible in the construction of AA plants, Fe3+-ions show the opposite influence towards MAA. here the presence of Fe3- shows a stabilizing effect.