Natural rubber was reinforced with a high loading of a cardanol-formaldehyde resin prepared from cashew nut shell liquid. Cardanol-formaldehyde resins, both resoles and novolaks, were synthesized from cardanol, which was extracted from cashew nut shells. This was done by the condensation polymerization of cardanol and formaldehyde in the presence of base and acid catalysts. The cardanol-formaldehyde resole with the highest yield (ca. 75%) was prepared with a formaldehyde/cardanol molar ratio of 2.0 at pH 8.0 and 90 degrees C for 8 h. The cardanol-formaldehyde novolak with the highest yield (ca. 80%) was prepared with a formaldehyde/cardanol molar ratio of 0.8 at pH 2.2 and 100 degrees C for 7 h. Fourier transform infrared and C-13-NMR were employed to characterize the chemical structures of the obtained cardanol-formaldehyde resins. The resins were compatible with natural rubber in various formulations. The cured behaviors of natural rubber blended with the cardanol-formaldehyde resole and novolak resins were investigated. The cured behaviors of cardanol-formaldehyde resole and cardanol-formaldehyde novolak samples were different, reflecting differences in their chemical reactivities. Furthermore, the incorporation of cardanol-formaldehyde resins into natural rubber provided significant improvements in mechanical properties such as the hardness, tensile strength, modulus at 100 and 300% elongation, and abrasion resistance. However, the elongation at break and compression set of the blends decreased as expected. (c) 2007 Wiley Periodicals, Inc.