A feasible mechanism for catalyst- and template-free group III-nitride nanorod growth by hydride vapor phase epitaxy (HYPE) is proposed. The mechanism is composed of random nanoparticle nucleation from stable gas-phase oligomers and subsequent directional growth along the c-axis. A combined study of equilibrium analysis and computational thermochemistry was employed to determine the optimum growth conditions-growth temperature and Cl/group III ratio-based on the proposed mechanism, and the Computed values showed good agreement with reported experimental results. The involvement of a group III trichloride as a key species in the proposed mechanism required the Cl/group III ratio to be similar to 3 according to stoichiometry. A higher Cl/group III ratio led to etching of the solid phase and a lower ratio favored two-dimensional film growth instead. The zone of GaN and InN nanorod growth by HVPE was shown to lie in the vicinity of the growth-etch transition. A two-temperature approach, employed in GaN nanorod growth, was supported by the deconvolution of two conflicting kinetic and thermodynamic constraints in terms of growth temperature: a high-temperature region for GaCl3 formation that is kinetically limited at low temperature and a low-temperature region for GaN nanorod growth without GaN etching that is thermodynamically favorable in a chlorinated environment at high temperature. The temperature for AlN nanorod growth by chemical vapor deposition using AlCl3 and NH3 was limited only by the thermodynamic constraint of ammonia adduct (Cl3Al:NH3) formation. (c) 2008 Elsevier B.V. All rights reserved.