NOVELTY - In a method for filling pressurized gas into a reservoir (11, 12) by compressing gas from a first source (1) to fill the reservoir directly or via buffer container(s) (2, 3) using energy from a working fluid (4), heat exchange is carried out between the compressed gas and the working fluid.    USE - Especially for filling hydrogen into the reservoir of a vehicle (claimed), e.g. a vehicle operated by a fuel cell or an internal combustion engine adjusted to run on hydrogen.    ADVANTAGE - Excessive heating of the gas due to rapid compression during filling (which is a potential cause of damage to the reservoir material, especially in the case of rapid filling) is controlled by the heat exchange between the compressed gas and the working fluid. The energies for compression of the gas and for cooling is carried by the same fluid. No electrically operated refrigeration devices are required, so that electricity consumption is minimized.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for apparatus for the process, comprising source(s) (1) of gas under a first pressure, a compressor (6) actuated by a working fluid (4) to compress the gas from the source, a source (40) of the working fluid, a gas compression controller (6) and (as novel feature) heat exchanger(s) (9) for exchanging heat between the compressed gas and the working fluid.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of one form of the filling apparatus.    First gas source (1)    Buffer container (2, 3)    Working fluid, consisting of nitrogen (4)    Cold/heat carrier or accumulator (5)    Gas compressor (6)    Line by-passing first heat exchanger (7)    Line with first heat exchanger (8)    Heat exchanger (9, 16)    Gas reservoir (11, 12)    Connecting lines (13, 14)    Valves (18, 1, 51, 52, 53, 56, 61, 76, 86, 131, 132, 135, 145, 141, 142, 203, 204)    Safety outlets to the atmosphere (21, 22, 23, 134, 144, 202)    Pressure sensors (26, 31, 32, 33, 96, 133, 143)    Compressor outlet line (36)    Working fluid source, consisting of liquid nitrogen reservoir (40)    Particle filter (46)    Temperature sensor (66)    Compressor feed line (101)    Heat exchanger supply line (116)    Coupling systems (136, 146)    Branch line (201)