Al substituted Li1+xAlxHf2-x(PO4)(3) (x=0.0 similar to 0.5) solid electrolytes were prepared by conventional sintering (CS) and spark plasma sintering (SPS), and the Li+ ion conductivity of the sintered pellets was examined using an impedance analyzer. All the samples exhibited a rhombohedral NASICON-type structure. SPS improved the densification compared to CS, and A1 substitution further increased the sample density resulting in a maximum density of similar to 90%. The Li+ ion conductivity increased in proportion to the sample density and the highest conductivity of 1.1 x 10(-4) Scm(-1) was found in the Li1.5Al0.5Hf1.5(PO4)(3) SPS specimen, which is comparable to the best value reported elsewhere. The activation energy of the total conductivity (bulk+grain boundary) reduced from 0.39 to 0.28 eV with Al substitution. The enhanced conductivity was discussed in terms of the densification, bottleneck size, and lithium ion content. (c) 2005 Elsevier B.V. All rights reserved.