Academic and industrial research groups are currently working to obtain high-quality transparent ultralow-refractive-index and ultralow-dielectric-constant thin films, as they are important for designing smart devices and systems for microoptics and microelectronics (for example, multilayer structures, optical resonators, photonic crystals, Cu interconnects, insulating layers and so on)(1). Here, we report a robust and simple procedure to prepare highly porous and resistant semicrystalline magnesium oxyfluoride thin films through liquid deposition followed by a flash and short thermal treatment. These films are water insensitive, mechanically resistant (E = 1 GPa), exhibit high optical quality and have an ultralow-refractive index (n(700nm) = 1.09) and an ultralow dielectric constant (k(100 kHz) = 1.6) which are required for the above-mentioned applications. Moreover, the process used to prepare these nanomaterials is well suited for industrial production on larger surfaces.