The reactions between epoxidized natural rubber (ENR) and a low-molecular weight palm oil-based alkyd (A1) have been investigated. Experimental results (FTIR and toluene swelling tests) showed that the alkyd having both hydroxyl and carboxylic groups could react with the epoxide groups of ENR at ambient temperature to cause crosslinking. To establish the predominant reaction, alkyd A1 was chemically modified to vary the amount of hydroxyl and carboxylic groups. A1 was treated with maleic anhydride under two different temperatures of 130 and 185 degrees C. At 130 degrees C, the anhydride has reacted partially with the hydroxyl groups to produce alkyd A2 with higher carboxylic content and lower hydroxyl content. On the other hand, at 185 degrees C, the anhydride has reacted completely to produce alkyd A3 with similar carboxylic acid content as A1 but lower hydroxyl content. Subsequent reactions of A2 and A3 with ENR under similar conditions have demonstrated that the predominant reaction with epoxide groups was due to the carboxylic groups from the fact that A2 could form higher amount of crosslinkages than A3, which has lower carboxylic content similar to A1. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 1503-1509, 2011