Plasma polymeric hydrogenated amorphous carbon (a-C:H) coatings have been found to be a promising material for use in surface transverse acoustic wave (STW) resonator employed as humidity sensor. The tailoring of coating properties has been easily obtained by variation of CH4 percentage in CH4-Ar mixture that enables to tune the response of polymer-coated STW resonator. Specifically, the optimum sensing characteristics of the STW resonator has been exhibited by coating obtained at 50% of CH4 percentage. The correlation between the device sensitivity and the material properties, as determined by small angle X-ray scattering (SAXS), X-ray reflectivity (XRR), ellipsometry and Fourier infrared spectroscopy (FTIR) measurements, permits us to highlight the sensing mechanism of the active a-C:H coatings. The porosity has been considered the main effect on the enhancement of the sensor response. (c) 2004 Elsevier B.V. All rights reserved.