The invention relates to a method of starting up a fuel cell system comprising a reformer (10) and a fuel cell stack (12), the reformer receiving during a first start-up phase a supply of oxygen and fuel with a first air ratio λi characterizing the fuel/air ratio, the reformer receiving during a second start-up phase a supply of oxygen and fuel with a second air ratio λ2 characterizing the fuel/air ratio, the first fuel/air ratio λ1 being larger than the second air ratio λ2 (λ1>λ2) and the fuel cell stack receiving a supply of reformate (18) generated in the reformer during the first and the second start-up phase.In accordance with the invention it is provided for that the transition from the first start-up phase to the second start-up phase is monitored by sensing a voltage furnished by the fuel cell stack (12).The invention relates furthermore to a fuel cell system.