The well known axial coordination capabilities of dirhodium(II,II) complexes towards Lewis bases have been exploited for the development of functional molecular materials for selective ammonia monitoring. On the basis of literature data and experimental evidence, the [Rh2(form)4] (1) (form = N,N′-di-p-tolylformamidinate anion) complex has been selected, among a series of dirhodium(II,II) derivatives, as the most suitable for this aim. By exposure to gaseous ammonia, 1 readily reacts, both in solution and in the solid state, with the gas to afford the corresponding ammonia-axial adduct. The reaction is reversible and is accompanied by a significant color change from dark yellow-green to red-brown due to the adduct formation, as confirmed by RGB, UV-vis and FT-IR spectroscopic investigations. Solid state UV/vis spectra reveal that while ammonia coordination occurs steadily, as judged by the blue shift observed for the λmax (470 for 1vs. 430 nm for the adduct), its complete release, in free air, takes more than a week. The potentiality of 1 for the selective ammonia detection in the headspace of aqueous ammonia solutions has been examined by QCM investigations. The dynamic response obtained from such studies is consistent with a reversible binding process occurring at the layer surface, which is operating once a fraction of the analyte is “irreversibly” bound to the sensor layer. The QCM response shows an excellent linearity in a wide concentration range (1–30 wt%) without any saturation effect up to high ammonia levels and a high selectivity to ammonia among a wide range of interfering gases.