The structural changes due to post-ash and post-ash treatments on chemical vapor deposited ultra-low-κ (ULK) SiOCH dielectric films were studied by Auger electron spectroscopy (AES) and Fourier transform infrared spectroscopy (FTIR). Changes in the ULK layer with respect to the carbon content were analyzed. For the application of different plasma gases for photoresist removal and further post-clean and anneal treatments first a reduction of carbon was observed. Using AES it was found that the carbon was removed up to ∼140 nm. Accompanied with the carbon loss a modification of chemical bonds was observed with FTIR, whereas the analysis of FTIR spectra was improved by means of chemometric methods. A principle component analysis was applied for qualitative analysis, which focuses on changes of infrared vibration peaks. This provides a fast assessment of chemical bond modifications. A partial least square regression was used to correlate the carbon loss with the infrared spectra. It is shown that the regression method allows a prediction of the carbon loss. For both methods the applicability and their limitations with respect to FTIR spectra are discussed.