The reaction layer formed on a silicon substrate in a CF4 rf plasma and in its admixtures with hydrogen and oxygen has been investigated by Fourier transform infrared phase-modulated spectroscopic ellipsometry (FTIR-PMSE). The results are compared with x-ray photoelectron spectroscopy spectra of the surface. It is shown that by using FTIR-PMSE, a significant gain in sensitivity is obtained as compared to ordinary reflection absorption measurements. In the case of a CF4 plasma, a large difference in the reaction layer at rf and the grounded electrode has been found. Samples treated on the rf electrode have a thin reaction layer consisting of SiFx (x = 1, 2, 3, and 4). The reaction layer of samples treated on the grounded electrode also contain CFx species. In the case of a 50% CF4 50% H-2 plasma, a thick carbon layer is formed on both the rf and the grounded electrode. While the layer formed on the grounded electrode contains a high amount of CFx (x = 1, 2, and 3) bonds, that on the powered electrode does not contain as much. In the case of a 50% CF4 50% O-2 plasma, a thin siliconoxide layer is formed. On the grounded electrode, it takes slightly longer to build up, but the final reaction layer on rf and grounded electrodes is the same. Small amounts of SiF4 are also present in the layer.