The aim of this article was an explanation of the gold nanoparticle (AuNP) formation mechanisms which take place in a redesigned Ultrasonic Spray Pyrolysis (USP). Depending on the synthesis parameters of gold concentration in the precursor solution, gas flow and reaction temperature, we have previously obtained a combination of spherical, irregular and cylindrical AuNPs. Two parameters (gold concentration and gas flow) were determined to be the most influential on the shapes and sizes of the produced AuNPs. The effects of these two influential synthesis parameters were evaluated on controlling the formation mechanisms: Gold concentration (2.5 and 0.5 g/l Au) in the precursor solution of HAuCl4 dissolved in water and gas flows of (i) aerosol carrier gas N-2 (1.5, 3.0 and 4.5 l/min) and (ii) reduction gas H-2 (1.0, 1.5 and 2.0 l/min). Depending on the parameter conditions, the AuNPs are formed from a combination of the liquid/solid phase (Droplet-to-Particle mechanism, DTP), the gas phase (Gas-to-Particle mechanism, GTP) and from intermediate secondary droplets, formed from primary droplet explosions. Increasing the gas flow affected the evaporation of the solvent (water) and diffusion of the solute ([AuCl4](-)) in the aerosol droplets, which resulted in the formation of more uniformly shaped AuNPs and with narrower size distributions than before. With favorable parameter conditions increased control over AuNP synthesis with USP has been achieved. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V.