Abstract: The reactants in a chemical laser are mixed to effect a lasing action by expanding one reactant through a relatively large primary nozzle in such a way as to form an expanding reactant flow field whose free expansion is essentially two dimensional in character and occurs predominantly in directions parallel to a given plane (expansion plane) parallel to the flow path of the flow field, and injecting the other reactant into this expanding flow field in such a way that mixing of the reactants occurs by predominantly molecular diffusion of the second reactant into the first reactant flow field in directions transverse to the flow field expansion plane.