In this work, we investigated the possibidlxlity to control both gas-phase chemistry and silicon etching kinetics in C4F8 + O-2 + Ar inductively coupled plasma by changes in O-2/Ar, C4F8/O(2)and C4F8/Ar mixing ratios at the constant fraction of the rest component (50%), gas pressure (10 mTorr), input power (700 W) and bias power (200 W). The combination of plasma diagnostics and modeling tools allowed one: (a) to compare the effects of gas mixing ratios on both steady-state plasma parameters and densities of active species; (b) to figure out key processes which determine the fluorine atom formation/decay balance in each gas system; and (c) to analyze the differences in Si etching kinetics in terms of process-condition-dependent effective reaction probability. It was shown that the maximum changes in gas-phase chemistry take place in O-2-rich plasmas due to CFx + O/O(D-1) -> CFx-1O + F, CFxO + e -> CFx-1O + F + e and CFO + O/O(D-1) -> CO2 + F stepwise dissociation pathways. It was suggested also that the effective probability for Si + xF -> SiF(x)reaction may be controlled by either fluorocarbon film thickness (in C4F8-rich plasmas) or O atom flux (in Ar and O-2-rich plasmas) through the balance of adsorption sites on the etched surface.