Rubbers are prevented from ozone deterioration without discoloring or staining by the addition of a nitrile of the formula, ##STR1## wherein R.sub.1 and R.sub.2 are each hydrogen, C.sub.1 - C.sub.24 aliphatic, C.sub.5 - C.sub.7 alicyclic or aryl, or R.sub.1 and R.sub.2 may form together with the adjacent nitrogen atom a heterocyclic group which may further contain oxygen, and R.sub.3 and R.sub.4 are each hydrogen or lower alkyl.