NOVELTY - The dilution unit useful in a reformer and a fuel cell for the passive dilution of fuels (1) e.g. natural gas, benzene, ethanol and methanol with water, comprises a flow channel (5) for fuel formed as a phase (3) such as membranes and/or fabrics having a catalyst (4). The phase is coated and/or impregnated with the catalyst, is diffusion-permeable for gaseous oxidizing agents (6) e.g. oxygen or air and/or gaseous reaction products (7) e.g. carbon dioxide and is impermeable for the fuel and/or the fuel mixture.    USE - Dilution unit useful in a reformer and a fuel cell for the dilution of fuels or fuel mixtures with water, adjustment of a defined water content in fuels or fuel mixtures, dilution of alcoholic solutions in the food industry, conversion and/or regeneration of the fuels, where the fuel consists of hydrocarbons such as natural gas, benzene, diesel, kerosene, methane, ethane, propane, butane, hydrogen, biogas, bio-diesel and vegetable oils, and alcohols such as methanol, ethanol, denatured alcohols and/or drinking alcohol (all claimed).    ADVANTAGE - The dilution unit has simple structure and is capable of operating in a maintenance-free manner without any mechanical component or additional water tanks.    DETAILED DESCRIPTION - The dilution unit useful in a reformer and a fuel cell for the passive dilution of fuels (1) e.g. natural gas, benzene, ethanol and methanol with water, comprises a flow channel (5) for fuel formed as a phase (3) such as membranes and/or fabrics having a catalyst (4). The phase is coated and/or impregnated with the catalyst, is diffusion-permeable for gaseous oxidizing agents (6) e.g. oxygen or air and/or gaseous reaction products (7) e.g. carbon dioxide and is impermeable for the fuel and/or the fuel mixture. The membranes and/or the fabrics are fuel resistant plastics, ceramic materials, coated fabric, semi-permeable membranes, porous membranes and/or ion exchanger membranes. An INDEPENDENT CLAIM is included for a method for the passive dilution of a fuel or a fuel mixture with water.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic structure of a passive dilution unit of fuels e.g. natural gas and methanol.    Fuels (1)    Phase (3)    Catalyst (4)    Flow channel (5)    Oxidizing agents (6)    Reaction products. (7)