We demonstrate that (NH4)(2)Si0.5Ti0.5P4O13 is an excellent proton conductor. The crystallographic information concerning the hydrogen positions is unraveled from neutron-powder -diffraction (NPD) data for the first time. This study shows that all the hydrogen atoms are connected though H bonds, establishing a two-dimensional path between the [(Si0.5Ti0.5)-P4O132-](n) layers for proton diffusion across the crystal structure by breaking and reconstructing intermediate H-O=P bonds. This transient species probably reduces the potential energy of the H jump from an ammonium unit to the next neighboring NH4+ unit. Both theoretical and experimental results support an interstitial-proton-conduction mechanism. The proton conductivities of (NH4)(2)Si0.5Ti0.5P4O13 reach 0.0061 and 0.024 S cm(-1) in humid air at 125 and 250 degrees C, respectively. This finding demonstrates that (NH4)(2)Si0.5Ti0.5P4O13 is a promising electrolyte material operating at 150-250 degrees C. This work opens up a new avenue for designing and fabricating high-performance inorganic electrolytes.