The physical vapor deposition of boron triiodide on macroscopic tungsten substrates was studied by using a Volmer-type apparatus. Data obtained were treated by a modification of the method of Pound. Results showed that nucleation of BI3 on tungsten was preceded by multilayer adsorption. The critical concentration of adsorbed monomers required for nucleation was 1.0 × 1015 molecules/cm2. Adsorption was time dependent at low temperatures. A steady state between impingent and desorption fluxes occurred at temperatures equal to or precedes 160°K. The nucleation data are described, using a disk model for the critical nucleus. The following Gibbs free energies were determined: (a) for formation of the critical nucleus, 1700 cal/mole; (b) for surface-diffusion activation, 1900 cal/mole; (c) for desorption activation, 11700 cal/mole; and (d) the critical volume free-energy change, - 72 cal/cm3. Weaknesses in both the macroscopic thermodynamic and statistical mechanical theories together with alternative interpretations of nucleation data are discussed. © 1968.