A method for controllable increase of porosity of low-k silicon oxycarbide films (SiOCH), deposited by oxidation of 3-methylsilane, has been developed using etching of the SiOCH film by diluted HF solution. The modified SiOCH film is characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectrscopy, and ellipsometric porosimetry. It is found that the chemical composition of the modified SiOCH film remains almost the same during etching. No significant thickness loss is observed, while the pore radius and the film porosity increase with HF dip time. It was concluded that the increase of the pore radius is caused by isotropic etching inside the pores as well as at the film surface. The very low etch rate of SiOCH film by diluted HF and the large difference between the pore radius and the film thickness allows an increase in the porosity without significant thickness loss. This method is a way to prepare ultralow-k dielectric films with higher chemical stability as compared to oxide and silsesquioxane-based porous materials.