NOVELTY - Hydrocarbon is dehydrogenated by passing a reactant stream comprising a gaseous hydrocarbon through a reaction chamber, comprising a dehydrogenation catalyst (15), where the reaction chamber has an internal dimension of at most 2 mm.    USE - The invention is for dehydrogenation of hydrocarbon to produce hydrogen and unsaturated compounds. Hydrocarbons are e.g. 2-18, preferably 2-10C alkanes, isobutene, propane, ethane, or 10-15C alkanes such as could be used for making detergent alcohols.    ADVANTAGE - The invention achieves lost cost, less complex processes, and/or apparatuses, fewer moving parts, reducing or eliminating the need for interstage heating, capability of long time on stream without regeneration, durability, stability, low coking, desired performance such as conversion, selectivity, high liquid and gas hourly space velocities, ability to operate under a variety of conditions including temperature, pressure and reactant composition, and compactness.    DETAILED DESCRIPTION - Dehydrogenation of hydrocarbon comprises passing a reactant stream comprising a gaseous hydrocarbon through a reaction chamber, comprising a dehydrogenation catalyst, where the reaction chamber has an internal dimension of at most2 mm; and dehydrogenating the hydrocarbon within the reaction chamber to form a product stream comprising an olefin, where the method further comprises coke-reducing step(s) consisting of quenching the product stream as it exists the reaction chamber, passing the reactant stream and/or product stream over a passivated surface in the flow path (4) or when the hydrocarbon comprises at least 3C hydrocarbon, or passing the reactant stream into the reaction chamber at a liquid hourly space velocity (LHSV) of at least 4.    DESCRIPTION OF DRAWING(S) - The figure is a representational, cross-sectional view of a hydrocarbon dehydrogenation apparatus.    Process channel (2)    Flow path (4)    Reaction chamber wall (6)    Heating chamber (10)    Dehydrogenation catalyst (15)