The properties of carboxylated nitrile rubber crosslinked through ionic crosslinks with calcium oxide (CaO) have been studied at two reaction temperatures (100 and 150degreesC). These properties increase with the CaO content to reach an optimum level, after which they tend to drop again as the level of CaO increases further. The changes in the properties are directly proportional to the amount of carboxylate formed. At any CaO content, a rise in the temperature and the corresponding reduction in the reaction time have a favorable effect on the properties.