A temperature-modulated sensor for ammonia based upon two quartz microbalances (QMBs) has been developed. One sensor element is coated with the tris-diethylammonium salt of 4,4',4''-[benzene-1,3,5-triyl-tri(ethin-2,1-diyl)tribenzoic acid (SPCA) [D. Meinhold, Thesis, Institute of Organic Chemistry, Mining Institute of Technology, Freiberg/Saxony, Germany, 1999.], which serves to detect ammonia, but shows a cross-sensitivity to humidity. To compensate for the influence of the latter, a sensor element coated with poly[ethylene imine] (PEI) functions as a humidity sensor.