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This paper reports the behaviour of a parallel coupled band pass microstrip filter due to an Al₂O₃ thin film‐thick film overlay and the effect of the moisture ambient on the properties of the overlaid microstrip filter. The thickness of the initial thin‐film overlay affects the behaviour of the filter after thick‐film overlay. Moisture has the effect of lowering the transmittance drastically and shifting the pass band to the lower frequency end. The filter loses its band pass characteristics after a few moisture‐heat cycles, indicating irreversible change taking place in the overlay material. It is felt that the ageing aspects of the overlay material should be taken into account when using dielectric overlays for circuit protection and cross‐over insulation purposes.