Silanisation of quartz substrate surfaces with a mixture of two chlorosilanes, namely trimethylchlorosilane and 7-octenyldimethylchlorosilane, leads to sensitive coatings for volatile organic compounds (VOC) on surface acoustic wave (SAW) devices. In this way we created monolayers of molecular cavities engulfing the analytes according to host-guest chemistry directly on the device surfaces, and also confirmed the occurrence of such cavities by molecular modelling. We monitored the binding process of the silanes by using Fourier transform infrared (FTIR) spectrometry and atomic force microscopy (AFM). In order to increase the stiffness of the cavities, we crosslinked the terminal double bonds of the long spacers by heating the surface in the presence of a radical initiator. Compared to SAW delay lines silanised with trimethylchlorosilane, devices modified with the binary silane mixture lead to substantially higher frequency shifts when exposed to solvent vapour streams. Nearly instantaneous responses can be observed, which e.g. allows xylene detection down to a few ppm.