The present disclosure relates to a process for treating carbon fiber, the process comprising: a) exposing one or more carbon fibers to plasma comprising carbon dioxide, and/or a reactive species thereof, thereby forming one or more pre-treated carbon fibers; and b) exposing the one or more pre-treated carbon fibers formed in step a) to a further surface treatment, thereby forming the treated carbon fiber. The present disclosure also relates to a composite material comprising one or more treated carbon fibers obtained by the process described herein and a matrix resin.