A theory of trapping gaseous reactants by aerosol particles is developed for arbitrary regimes of reactant transport. The dependence of the trapping efficiency on the particle size is found as a function of sticking probability of the reactant molecules to the particle surface. The key point of this consideration is the solution of the transport equation in the free-molecule zone (where the collisions between the reactant molecules and the molecules of the carrier gas can be ignored) and further matching the reactant concentration profiles at the interface separating the free-molecule and diffusion zones. The flux conservation allows for the formulation of the boundary condition that determines the reactant surface concentration. The latter depends on the total flux of the reactant and thus the trapping efficiency of the reactant molecules occurs to be dependent on the nature of in-particle chemical processes. The first-order chemical reaction serves as a good example of such dependence, where all characteristics of the trapping efficiency can be found analytically.