Over 20 different molecules and radicals have so far been detected in interstellar space. The number of such molecules and radicals and their frequently complicated structure raise the question of the processes leading to their formation. Diatomic molecules and radicals could arise in gas‐phase reactions. Formation of polyatomic molecules can be explained in terms of reactions proceeding on the surface dust particles and possibly involving interstellar radiation. The author demonstrates, with the aid of a model, that the molecular abundances and distributions within dust clouds in certain regions can be explained by photocatalytic reactions.