Abstract: A passive process for selectively sensing analyte molecules in a fluid by reflection of acoustic waves from a sensing surface of a bulk accoustive wave quartz sensing device to which analyte molecules bind is provided. The sensing surface has a plurality of receptors for which analyte molecules have an affinity. The process has the following steps:i) contacting a fluid in which analyte molecules are suspected with the sensing surface so that acoustic waves are reflected from the sensing surface;ii) directing acoustic waves at the sensing surface; andiii) detecting a change in a characteristic of acoustic waves reflected from the sensing surface due to analyte molecules binding the receptors. The change in a reflected acoustic wave characteristic being measured by electrical means detecting values which are related to the amplitude and phase of the reflected acoustic waves.