This paper reports an experimental study on the stability, coagulation and diffusion of molecular clusters and ultrafine particles generated from organic vapors by corona ionizers. Upon leaving the ionizer, particles are made to flow within several types of vessels: depending on the specific geometry of the flow system, clusters either coagulate into large particles or are deposited on the walls. Particles larger than 4 nm and molecular clusters penetrate through a wire-screen type diffusion battery, but particles in the size range between 2 and 4 nm are collected. Among the organic compounds tested (aromatics, alcohols, ketones and others), only aromatic compounds appear to yield unstable clusters which grow into detectable particles (>2 nm) through Brownian coagulation. The other compounds either do not undergo the gas-to-particle conversion process or are too small to be detected. Furthermore, the presence of moisture seems to be of fundamental importance in the particle generation phenomenon. The addition of alcohols to the vapor mixture inhibits particle formation.