Treating an uncured resin matrix composition to impart energy to the resin matrix composition enables a stable Tg to be achieved whereby the resin matrix composition may be stored and has a low tack enabling subsequent processing and handling. The invention provides a method of achieving a stable Tg without the resin matrix composition starting to cure and a method of determining the treatment regime by which a resin matrix composition with a stable Tg may be obtained. The resin matrix composition may be fresh or reused uncured resin matrix composition and may contain fibrous reinforcement.