Isotropic etching of silicon in HF-based solutions is expected to be controlled by the diffusion of fluoride to the silicon surface. In order to gain quantitative understanding of the process, we studied etching of silicon in HF/HNO3/H2O via circular mask openings and compared the results with the theoretical expectations. The cavity edges due to etching under the mask were analyzed with a high precision by processing the optical microscope images. Dependence on the etching time and opening size was investigated. A small anisotropy was observed in perfect agreement with the crystal orientation symmetry. A weak effect of free convection induced by the gravitation was resolved. Importance of careful temperature control is stressed. The observed time dependence agrees perfectly well with the theoretical prediction. It was verified with 1% precision. Dependence on the opening size predicted theoretically is not fully supported by the experiment. There is a small (4%) but clearly observable deviation from the theory. It is demonstrated that both time and opening size dependencies can be predicted with 1% precision if one introduces an effective diffusion “constant��? that changes slightly with the opening size.