PURPOSE: To modify and improve a rubber containing an unsaturated carbon bond without causing side reactions such as gelation and MW lowering of the rubber, by reacting the rubber with an oxomalonic ester in the presence of a Lewis acid.

CONSTITUTION: A rubber containing an unsaturated carbon bond (e.g., natural rubber or polyisoprene rubber) is modified by reacting it with an oxomalonic ester of the formula, wherein R1 and R2 are each a 1W20C linear aliphatic group, a side chain-containing aliphatic group, an alicyclic group or an aromatic group, in the presence of a Lewis acid (e.g., SnCl4). The amount of the oxomalonic ester used is usually 0.01W20pts.wt., preferably 0.1W5pts.wt., per 100pts.wt. unsaturated rubber. The amount of the Lewis acid used is usually 0.01W5mol, preferably 0.05W2mol, per mol of the oxomalonic ester.