Electron cyclotron resonance (ECR) plasma etching of Si in Cl2 has been studied from the viewpoint of plasma chemistry. Experiments were performed over a wide pressure range (0.2–10 mTorr), using a divergent magnetic-field ECR plasma reactor supplied with 2.45-GHz microwaves; a floating electrode or substrate holder was located ∼30 cm downstream (B≈150 G) of the 875-G ECR resonance region, and samples of polycrystalline Si were etched with no additional wafer biasing. Several diagnostics were employed to characterize the plasma around the wafer position, including two-photon laser-induced fluorescence (LIF) for detection of Cl atoms and Fourier transform infrared (FTIR) absorption spectroscopy for etch products or SiClx (x=1–4) molecules. The Si etch rate behavior obtained is interpreted in terms of a modified adsorption-reaction-ion-stimulated desorption process model based on these diagnostics.