A basic principle for selecting inorganic sealing materials for dense ionic-conducting ceramic membranes is described for high temperature permeation/reaction experiments. Based on this principle ceramic-glass composite seals consisting of the Pyrex glass and the ceramic powder of the membrane were developed and successfully used to seal a number of different dense ceramic membranes at high temperatures. The ceramic-glass composite seal is typically composed of 40-50 wt.% membrane material powder, 20-50 wt.% Pyrex glass and 5-20 wt.% additive such as sodium aluminate and boron oxide. The properties of ceramic-glass composite seal can be tailored to obtain suitable wettability, viscosity, chemical inertness, thermal expansibility, and bonding strength for good sealing results. A success rate for sealing these ceramic membranes of nearly 100% is possible using the ceramic-glass composite recipe if the correct sealing procedure, including seal paste preparation, is carefully followed.