In this work, the effect of alkaline-earth species used in phosphate glasses on the mobility of proton carriers was investigated. Initially, 35NaO(1/2)-5RO-3NbO(5/2)-3LaO(3/2)-2GeO(2)-2BO(3/2)-50PO(5/2) (R: Mg and Ba) glasses were subjected to the electrochemical substitution of Na+ ions with H+ at intermediate temperatures, producing proton conductors with high proton carrier concentrations (>8 x 10(21) cm(-3)). The mobility of the proton carriers in the Ba glass was higher than that in the Mg glass. We studied the origin of this phenomenon in terms of the O-H and P-O bonding features using infrared spectroscopy and X-ray photoelectron spectroscopy, respectively. The higher mobility of proton carriers in Ba glass was attributed to weaker O-H bonding, which originated from the highly ionic character of Ba-O bonding compared with the comparatively covalent Mg-O bonding. Our findings indicate that a higher mobility of proton carriers can be achieved for glasses containing less electronegative glass network modifiers. The application of this strategy would be beneficial for the development of proton-conducting glass electrolytes for intermediate-temperature fuel cells.