Homogeneous nucleation of NH4Cl from NH3 and HCl was studied. When no ions or neutral radicals were present, the nucleation rate increased exponentially with increasing supersaturation above a threshold value. Increasing relative humidity acted so as to reduce the threshold. The curves obtained are consistent with the expression for the free energy of critical embryo formation based on the reaction NH3+ HCl=NH4Cl. Water vapor does not enter into the reaction but acts as a catalyst. When ions and radicals were added to the reactants, the nucleation rates were enhanced, and the thresholds were reduced; negative ions were the most efficient in producing these effects. An inflection was also observed in these cases, and the general shape of the curves log (nucleation rate) versus concentration of reactants changed, indicating a change in the reaction mechanism.