ABSTRACT. Formation of nanometer-sized particles in NH,ISOJH,OIAir mixtures by a-ray irradiation was studied experimentally to evaluate the effect of NH, gas on the gas-to-particle conversion process. The total particle number concentration, the charged-particle fraction, and the electrical mobility distribution of particles generated in the ionization chamber were measured at various NH,, SO,, and H,O vapor concentrations and residence times of the gas mixture. Particle generation was enhanced in the presence of NH,, but the enhancement depends greatly on the SO,, NH,, and H,O concentrations; particle number concentrations at NH, mixing ratios of 0.71-3.6 ppm were 2-4 times higher than those in the absence of NH,, but were constant in this range of NH, concentration. The charged-particle fraction decreased with the addition of NH,, which indicated that ion-induced nucleation was decreased and homogeneous nucleation was enhanced. To evaluate the number concentrations of gaseous products, kinetic equations for the vapor species were solved on the basis of chain reactions initiated by OH radicals.