In the present study, synthesis and decomposition of zinc hydroxynitrates, i.e., Zn(NO3)(OH)center dot H2O, Zn-5(NO3)(2)(OH)(8)center dot 2H(2)O, and Zn-3(OH)(4)(NO3)(2) into ZnO nanoparticles were investigated at supercritical conditions by combination of Taguchi experiment design method, analysis of variance (ANOVA), and complementary verifying experiments. First, the effect of several important parameters such as temperature, residence time, and initial nitrate salt concentration and its pH on "purity" of synthesis particles (e.g., ZnO) was studied using an orthogonal experiment design methodology coupled with instrumental analysis such as powder X-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, and thermo-gravimetric analysis. Then ANOVA was performed on the data and the best conditions for the synthesis of pure ZnO nanoparticles were attained. The second aim was the elucidation of formation and decomposition mechanism of hydroxinitrates by combining the statistical results and additional experiments which influential parameters were investigated independently. It was found that the initial pH of zinc nitrate alongside temperature had the most effects on formation of Zn-5(NO3)(2)(OH)(8)center dot 2H(2)O as the first product of the formation reaction, while at higher temperatures it was converted to Zn-3(OH)(4)(NO3)(2) which in turn decomposed to ZnO nanoparticles as an ultimate product.