By mixing certain clay mineral samples into finely divided EMD (electrolytic manganese dioxide) and by using the mixtures as the solid electrolyte of a galvanic potential type humidity sensor, which consisted of a Pt/EMD/Cu or AZ system, it was found that the clay minerals exhibited a new function to markedly improve the performance of the EMD sensing element, which otherwise responded only to high-humidity regions. The response time profiles of the galvanic potentials produced by the clay-admired sensor elements were different from the EMD’s. It was observed that different effects took place depending on the nature of the additives, and kaolinite and muscovite enhanced the humidity-sensing characteristics of EMD, showing good linearity and fast response. A mechanism for the enhancement of humidity-sensing characteristics was proposed, taking into account the role of interlayer water molecules present in the EMD-additive system.