We present experimental studies on the deposition and characterisation of ZnS/Ag/ZnS low-e coatings carried out to assess their optical performance and stability. Theoretical predictions of the coatings optical properties were combined with measured emissivity (epsilon) values to derive a figure of merit (eta) defined as the ratio of luminous transmittance over emissivity. A coating consisting of 37.0 nm ZnS/21.5 nm Ag/37.0 nm ZnS was found to maximise eta. Long-term experiments on the produced ZnS/Ag/ZnS coatings, under solar irradiation have revealed that a moderate vacuum (10(-2)-10(-3) mbar) is required to avoid degradation of the him properties. Four-layer ZnS/Al/Ag/ZnS and five-layer ZnS/Ag/ZnS/Ag/ZnS coatings were produced and tested for thermal stability. They were both found to withstand heating at 300 degrees C for 3 h. The four-layer structures however, present low transmittance in the visible, due to absorption by the Al layer.