The mass sensitivity of an AlN thin film resonators operated in air at 6-8 GHz has been investigated in theory and experiment. The 30 x 30 mu m wide resonators included a 180-300 nm thick, highly oriented AlN thin film, bottom and top electrode, and an acoustic reflector made of AlN/SiO2 multilayers. The top electrode was loaded by defined polymer layers to test sensitivity and simulation model. The impact of type and thickness of the top electrode was simulated. Ten to twenty nanometers thick PMMA coatings served to demonstrate sensitivity with a gettering gel in form of spontaneous adsorption of acetone from the vapour phase. Finally, a self-assembled monolayer of 11-mercaptoundecanoic acid was formed on a Pt top electrode for the demonstration of a sub-monolayer sensor in air. In both cases, pico-grams could be detected with a sensitivity up to 1000 m(2)/kg. (c) 2005 Elsevier B.V. All rights reserved.