The technique described here combines elastomeric materials and thermosetting resins, specifically natural rubber and cardanol-formaldehyde (CF) resins, to improve mechanical properties (such as toughness) and thermal properties (such as high-temperature resistance). The natural product cashew nut shell liquid was used to obtain the cardanol required, which was then used to prepare cardanol-formaldehyde resole and novolak resins. This was done using a condensation polymerization with formaldehyde in the presence of base and acid catalysts. The resins were found to be compatible with natural rubber in various formulations. The cure behavior of natural rubber blended with either the CF resole or novolak was investigated. The blended materials showed improved mechanical properties and good thermal stability, with an increase in the resin-hardener ratio leading to increases in tensile strength and thermal stability. Aging studies showed that these modified elastomers also had improved oxidative stability.