Our earlier work on the formation of particulate NH4N03 in the NH3-03 reaction at 25OC is extended to include air as a diluent and H 2 0 vapor as an additive. More extensive data at different values of [NH3]/[03]O were obtained also, where [ 0 3 ] 0 is the initial 0 3 concentration. In our earlier work we concluded that NH4NO3 vapor was dissociated to NH3 + H N 0 3 and that the HN03 was removed by diffusion to the walls with a rate coefficient kdiff = 0.4 min-' or by condensation on the suspended particles. Particles were nucleated by 8 NHs-HNO, pairs when their concentration product reached 5.8 X 102' molecP/cm6 with a rate coefficient knuol of 6.2 X ~m ~~/ m i n and removed by coagulation with a rate coefficient koosg of 1.3 X 10-7 cm3/min. A corrected calculation modifies the number of pairs required to 6-7 with a correspondingly changed value of With the more extensive data of the present study the indications are that the vaporphase NH4NO3 monomer is not dissociated and that its diffusion constant for loss to the walls varies between 0.3 and 0.9 min-I for different reaction conditions. Nucleation occurs when the NH4N03 vapor concentration reaches 1.0 X 10l2 molec/cm3 via rNH4NOdg) -+ (NH4N0dP(s) where r is 9 and the nucleation rate coefficient knucl is 3 X 10-108 cmZ4/min. With 5.0 or 9.5 torr of HZO vapor present, there is an excess of particles produced over that expected from this rate coefficient, indicating an additional nucleation step in which H20 vapor participates directly to produce a hydrated salt. The coagulation coefficient of (1.87 & 0.14) X 10-7 cm3/min found here is in good agreement with that found previously. knuel.