The ionic conductivity of LixB1-x/3PO4 is investigated in terms of defect models with a lithium doping level of 0 < x < 0.3. The crystalline solid electrolytes, LixB1-x/3PO4, are synthesized by a soft-chemical route by using H3BO3, H3PO4, LiOH.H2O. To determine the changes in ionic conductivity and activation energy of LixB1-x/3PO4 with lithium content x, AC-IS measurements are performed at temperatures between 25 and 90 degreesC. The maximum ionic conductivity is 3.74 x 10(-6) S cm(-1) at room temperature for x = 0.10, and the activation energy increases from 0.3039 to 0.3385 eV with increasing lithium content. (C) 2003 Elsevier Science B.V. All rights reserved.