Reactive ion etching (RIE) of p-type 2-3 †cm resistivity silicon (100) was characterised using Photoreflectance (PR), Rutherford Backscattering Spectrometry (RBS) and Spectroscopic Ellipsometry (SE). Isochronal (5 minutes) etching was performed at various DC etch biases (0-500V) using a SiCl4 etch chemistry. The substrate etch rate dependence on applied bias was determined using mechanical profilometry. A distinct shift in the A3–A1 Si transition and significant spectral broadening of the room temperature PR spectra was observed as a function of etch bias. Photoreflectance results are correlated with RBS, SE and etch rate analysis. It is demonstrated that the PR spectra reflect a complex, competitive, plasma-surface interaction during the RIE process.