Processes for preparing a carbon product having an organic group attached to a carbon material. The carbon material is selected from graphite powder, a graphite fiber, a carbon fiber, a carbon cloth, a vitreous carbon product, and an activated carbon product. In one process at least one diazonium salt reacts with a carbon material in the absence of an externally applied electric current sufficient to reduce the diazonium salt. In another process at least one diazonium salt reacts with a carbon material in a protic reaction medium.