This work features the study of kinetics and mechanisms of redox reaction between [AuCl₄]⁻ and hydrazine sulfate, as well as the kinetics of gold nanoparticles (AuNPs) formation. UV–Vis spectrophotometry and dynamic light scattering (DLS) method were used to determine the influence of reductant concentration on the rate of Au(III) ions reduction and Au NPs formation. It was found that the reaction mechanism constitutes several steps. Homogeneous, bimolecular reduction of Au(III) complex ions comprise the first step. The second step consists of autocatalytic reduction of Au(I) ions to a metallic form and triggers the nucleation and autocatalytic growth of AuNPs. Using the modified Finke–Watzky model and the obtained kinetic data, the following values of the rate constants were determined: k₁=1.188 (±0.019)M⁻¹s⁻¹, k₃=3.87 (±0.26)10⁻²M⁻¹s⁻¹ and k₄=7.53 (±0.21)10⁵M⁻²s⁻¹. The study on DLS and transmission electron microscopy (TEM) indicates that the growth of AuNPs is a result of the autocatalytic redox reaction followed by the reaction-limited Ostwald ripening. The rate laws describing homogeneous reduction of gold(III) ions and the evolution of the hydrodynamic radius of AuNPs were also determined.