<p>We prepared ZrO₂ thin films by spin-coating from Zr(OC₃H₇^<i>n</i>)₄ and ZrOCl₂·8H₂O solutions, and investigated the influence of relative humidity (R.H.) during spin-coating on their microstructure and porosity. The Zr(OC₃H₇^<i>n</i>)₄ solutions contained either a peptizing agent (HNO₃) or a chelating agent (CH₃COCH₂COCH₃, acac), while the ZrOCl₂ solutions with or without acac were used. Spin-coating was performed on Si(100) substrates under various R.H. conditions, and the resulting precursor films were heated up to 500 °C at a rate of 5 °C/min. Field-emission scanning electron microscopic observation was made, and ellipsometry was employed to obtain refractive indices, from which porosities were calculated. The ZrO₂ films from Zr(OC₃H₇^<i>n</i>)₄ solution with HNO₃ exhibited relatively dense microstructures and porosities as low as ~25% when deposited at R.H. as low as 20%, while they developed nanoscale crevices and had increased porosities up to ~47% when deposited at R.H. above 50%. When Zr(OC₃H₇^<i>n</i>)₄ solution contained acac, on the other hand, the films maintained dense microstructures and porosities as low as 20% even when deposited at R.H. as high as 70%. In contrast, the films from ZrOCl₂ solutions consistently showed dense microstructures and porosities as low as 13%, regardless of R.H. during deposition or acac addition. These results suggest that, to obtain dense ZrO₂ films with low porosity, one should either use zirconium salts as precursors or incorporate chelating agents in zirconium alkoxide solutions, both of which may effectively suppress the hydrolysis by ambient water vapor.</p><p></p>