NOVELTY - A fuel cell system comprises a fuel cell; a hydrocarbon fuel processor (400) for generating hydrogen (409) for use in the fuel cell; and a hydrogen buffer (420) for storing a portion of the hydrogen produced in the fuel processor for use by the system during start-up of the system or when required by the fuel cell during the course of operation of the system. The fuel processor is a steam reformer, preferably a pressure swing reformer (PSR). The system further comprises a hydrogen separator (408) comprising at least two membrane stages operating in series.    USE - In confined space applications such as on-board vehicle applications.    ADVANTAGE - The fuel cell system is capable of providing hydrogen fuel to the fuel cell in a manner that provides rapid start-up. The fuel cell system incorporating a hydrocarbon fuel processor that permits the fuel cell system to perform any rapid transients.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel cell; a hydrocarbon fuel processor (400) for generating hydrogen (409) for use in the fuel cell; and a hydrogen buffer (420) for storing a portion of the hydrogen produced in the fuel processor for use by the system during start-up of the system or when required by the fuel cell during the course of operation of the system. The fuel processor is a steam reformer, preferably a pressure swing reformer (PSR). The system further comprises a hydrogen separator (408) comprising at least two membrane stages operating in series. Each stage comprises a hydrogen-selective membrane. An INDEPENDENT CLAIM is included for method for supplying hydrogen fuel for a fuel cell involving generating hydrogen in a hydrocarbon fuel processor; storing a portion of the hydrogen produced in the fuel process; supplying the balance of the hydrogen produced in the fuel processor to the fuel cell; supplying stored hydrogen to at least one of the fuel cell during start-up; the fuel processor during start-up for burning and heating the components; and when the amount of hydrogen generated by the fuel processor is temporarily less than the operating demand of the fuel cell.    DESCRIPTION OF DRAWING(S) - The figure shows a diagrammatic view of fuel cell system.    Hydrocarbon fuel processor (400)    Shift reactor (406)    Hydrogen separator (408)    Hydrogen (409)    Hydrogen buffer. (420)