The reaction between Si and O-2 was studied in liquid He droplets at low temperature (T = 0.37 K) by monitoring the energy release during the reaction. Additionally, the reactions of Si atoms and clusters with the oxidation agents H2O and O-2 have been studied by mass spectrometry. It was found that Si atoms react fast with O-2 molecules. On the other hand, Si atoms and clusters do not react with H2O molecules. The energy released during the chemical reaction leads to the ejection of the products from small He droplets. In contrast, large He droplets (N-He > 20000) are capable of keeping part of the reaction products in their interior. The observation of SiO2 products with the mass spectrometer reveals that the He droplet can stabilize intermediate products in the exit channel.