A miniaturised piezoelectric mass sensor has been developed that displays a very high mass sensitivity, Sm,. The sensor Is based upon thin film resonator (TFR) technology,1 which Is used to produce a gold-coated thin film (5.5 μm) resonator (TFR) of piezoelectric aluminum nitride In the shape of 400-μm squares. At this thickness, the TFRs support a standing bulk acoustic wave in a longitudinal mode at frequencies of ~ 1 GHz. Coatings of poly(methyl methacrylate) and spontaneously adsorbed monolayers from alkanethlols (CH3-(CH2)nSH, n = 3, 17) were used for the evaluation of Sm. Results Indicated that the Sm(~-550 cm2/g) of the TFRs rivals those of the most sensitive mass sensors currently available, while offering such performance In a markedly smaller device. Coatings were also used to sorb a gaseous analyte (l.e. methanol) at the TFRs to test their potential as gas-phase sensors. Tests conducted using monolayers formed by the chemlsorption of a fluorlnated thiol (CF3(CF2)7(CH2)2-SH) and an acld-termlnated thiol (HO2C(CH2)15SH) at the gold electrodes demonstrated both the performance of the TFRs and a primitive manipulation of the sorption selectivity of the interfacial structure. Possible applications of the TFRs as mass sensors are examined based upon their high mass sensitivity and compatibility for device integration using semiconductor processing technology.