Highly tetravalent zirconium-ion-conducting solid electrolytes, Zr1+x/4TaP3-xSixO12, were developed by partially substituting the P5+ ions in the Nasicon-type ZrTa(PO4)(3) solid, with Si4+ ions having larger ionic size and lower valence. Among the samples prepared, Zr41/40TaP2.9Si0.1O12 (x=0.1) exhibited the highest Zr4+ ion conductivity of 3.04x10(-4) S cm(-1) at 600 degrees C.