The present invention relates to a method for heating a fuel cell system (1a; 1b; 1c; 1d) comprising at least one fuel cell stack (2) with an anode portion (3) and a cathode portion (4), and a reformer (5) upstream of the anode portion (3) for steam reforming using a fuel, the reformer (5) comprising a nickel-based catalyst, said method having the following steps: starting a heating process for heating the fuel cell system (1a; 1b; 1c; 1d) with a heating device (6) and conducting a carbon-containing fluid and conducting steam through the nickel-based catalyst of the reformer (5) during the heating process. The invention also relates to a fuel cell system (1a; 1b; 1c; 1d) which is designed to carry out a method according to the invention.