A study on low-refractive-index SiO2 antireflective (AR) coatings by a sol-gel method is reported. Variations in the properties of the coatings are related to the molar ratios of ammonia to deionized water being changed in the process of preparing the sols. From the performance test results, the optimal ratio of the reactants necessary to prepare low-refractive-index SiO2 AR coatings is determined. Of all the SiO2 AR coatings, the lowest recorded refractive index is 1.16 at a wavelength of 700 nm. The largest water contact angle is 121.2°, and the peak transmittance is 99.95% at a wavelength of 908 nm. Furthermore, the sol used to deposit the film with the lowest refractive index is stable because of the narrow size distribution of its constituent particles.