To provide a composition (e.g., a coating composition) useful for forming an organic semiconductor having excellent conductivity, solvent resistance, heat resistance, durability, and other properties, and an organic semiconductor formed with the composition.The composition comprises an aromatic polycarbonyl compound having a carbonyl group as a reactive site, and at least one aromatic reactive component selected from an aromatic polyamine having an amino group as a reactive site, and an aromatic heterocyclic compound having a plurality of unmodified α-carbon positions, as reactive sites, each of which is adjacent to a hetero atom of a heterocycl thereof. When the aromatic reactive component is an aromatic heterocyclic compound, the aromatic polycarbonyl compound is an aromatic polyaldehyde compound. Use of the composition in which at least one component of the aromatic polycarbonyl compound and the aromatic reactive component has not less than 3 reactive sites per molecule forms an organic semiconductor having a three-dimensional crosslinked structure.