Various types of solid electrolytes based on ammonium hydrogermanate in which the germanium site was partially replaced by phosphor were prepared by a hydrothermal method and the ammonia gas sensing characteristics were investigated by fabricating an ammonia gas concentration cell with the hydrogermanates. The EMF output of the response was greatly improved by forming a solid solution by the phosphor doping with a considerable enhancement of the relative density and also the mechanical strength of the solid electrolytes.