The reactions between polysilazane and hafnium-butoxide precursors are followed from the liquid, to the cross-linked, and finally to the ceramic state by infrared spectroscopy. We find evidence of formation of Hf-N bonds in the liquid state, and Hf-O-Si bonds in the cross-linked polymer state. Results from ab inito calculations for the ceramic state are presented. They show that Hf behaves like Si, forming bonds with C, N, and O in proportion of the Hf/Si ratio in the compounds. The average values of such bonds formed by Hf, relative to silicon, are 11.2, 17.8, and 24.4%, which is in reasonable agreement with the overall Hf/Si ratio in the samples (8%, 15% and 22%). It is concluded that it is appropriate to express the composition of these compounds in terms of the Hf/Si ratio. The X-ray data that show the emergence of weak diffraction peaks for monoclinic HfO2 when the Hf/Si ratio is 0.22, but not at lower concentrations of Hf [K. Terauds and R. Raj, J. Am. Ceram. Soc., 96, 2117-2123 (2013)].