Photolysis of CH2I2 in the presence of O3 has been proposed as a mechanism of new particle formation in coastal areas. We report here a comprehensive experimental chamber study of this system. Rapid homogeneous nucleation was observed over three orders of magnitude in CH2I2 concentration, down to levels representative of peak concentrations in coastal areas. After the nucleation burst, the observed aerosol dynamics was dominated by condensation of additional vapors onto existing particles and particle coagulation. The particles formed under dry conditions are fractal agglomerates with mass fractal dimension Df ~ 2-2.5. Higher relative humidity (65%) does not change the nucleation or growth behavior from that under dry conditions, but results in more compact and dense particles. Based on known gas-phase chemistry, only the gas-phase species OIO and I2O2 could produce the observed nucleation and aerosol growth. Chemical analysis of the aerosol using an Aerodyne Aerosol Mass Spectrometer reveals that the particles are composed mainly of iodine oxides but also contain iodine oxyacids, likely HIO3. The latter may be formed in the gas phase or by reactions in/on the solid particles.