Ti4+ ion substitution ranges for Zr4+ ion have been investigated in the Na1+xZr2SixP3-xO12 (NASICON) system synthesized by a conventional solid state reaction using inorganic compounds at the reaction temperatures of 1000, 1100 and 1200 degrees C. The Ti4+ ion substitution rate was greater in the phosphate-rich region than in the silicate-rich region. In the silicate end member, the Ti4+ ion substitution rate and the melting temperature decreased with increasing Ti4+ ion content of the starting materials. In the narrow range around x similar to 2.5, at low Ti4+ ion content and at low temperatures; the Na2ZrSi2O7 phase segregated. Essentially the same results, except for the Na2ZrSi2O7 phase, have been shown when the NASICON compounds were synthesized by a conventional sol-gel technique using metal alkoxides.