It is accepted that the macroscopic properties of functional plasma polymer films (PPF) are defined by their functional density and their crosslinking degree (chi) which are quantities that most of the time behave in opposite trends. If the PPF chemistry is relatively easy to evaluate, it is much more challenging for chi. This paper reviews the recent work developed in our group on the application of principal component analysis (PCA) to time-of-flight secondary ion mass spectrometric (ToF-SIMS) positive spectra data in order to extract the relative cross-linking degree (chi) of PPF. NH2-, COOR- and SH-containing PPF synthesized in our group by plasma enhanced chemical vapor deposition (PECVD) varying the applied radiofrequency power (P-RF), have been used as model surfaces. For the three plasma polymer families, the scores of the first computed principal component (PC1) highlighted significant differences in the chemical composition supported by X-Ray photoelectron spectroscopy (XPS) data. The most important fragments contributing to PC1 (loadings >90%) were used to compute an average C/H ratio index for samples synthesized at low and high P-RF. This ratio being an evaluation of chi, these data, accordingly to the literature, indicates an increase of chi with P-RF excepted for the SH-PPF. These results have been cross-checked by the evaluation of functional properties of the plasma polymers namely a linear correlation with the stability of NH2-PPF in ethanol and a correlation with the mechanical properties of the COOR-PPF. For the SH-PPF family, the peculiar evolution of chi is supported by the understanding of the growth mechanism of the PPF from plasma diagnostic. The whole set of data clearly demonstrates the potential of the PCA method for extracting information on the microstructure of plasma polymers from ToF-SIMS measurements. (C) 2015 Elsevier B.V. All rights reserved.