In this work, the photoacoustic method is employed to monitor the copolymer formation from ethylene vinyltrimethoxy silane and grafted vinyltrimethoxy silane on low-density polyethylene when cross-linked using water saturated vapour. Photoacoustic spectra at overtones of –CH2–, –CH3– and –OH showed that the cross-linking processes were more efficient when the samples were prepared at 80°C with the catalyst concentration in the range between 5% and 7%. Using different light modulation frequency procedure it was observed that –OH and –CH2– groupings were more concentrated near surface, showing a larger concentration gradient for –CH2– than –OH. In addition, the thermal diffusivity analysis did not present a tendency that could be used for understanding the cross-linking process, although, the average value of the product ρcp were 1.6 (J K cm−3) for the grafted polymer and 2.5 (J K cm−3) for the copolymer, respectively. It is suggested that grafted samples present a higher thermal conductivity.