Si anodic dissolution in acidic fluoride medium has been investigated in different regimes (porous silicon formation, electropolishing under stationary and oscillating current) by probe beam deflection (PBD) or the "mirage" technique. Evolution of deflection signal allows monitoring of dissolution processes both under polarization and at open circuit, providing for example an estimate of the oxide etch-back times during open circuit corrosion. The time evolution of deflection signal during current oscillations reveals components resulting from electrochemical film formation and chemical dissolution. The PBD technique shows larger etching rates for the less passivating film (low potential) and smaller etching rates for the better passivating film (high potential). Our observations are compared with those obtained from spectroscopic investigations of the oxide layer and their relevance for models of oxide formation, and dissolution is here discussed.