A synthesis of sulfonated graphene has been developed through a rational investigation of more than 8 different routes. The synthesis involves (1) the preparation of graphene through sequential oxidative exfoliation of graphite to graphene oxide and reduction with hydrazine, (2) the grafting of graphene with 4-diazoniobenzenesulfonate, leading to sulfonated graphene. The procedure features a good and adjustable level of grafting, a high reproducibility, fast filtrations, and a scalability on a gram-scale. The material displayed a high density of relatively strong acid sites along with robustness in its dry state, up to 200 C.