Electrical and dielectric properties, ionic transference numbers, thermal conductivity, and microhardness of Bi5.8PO11.2 (BPO) and Bi4Ge3O12 (BGO) single crystals are studied. Pr3+ and Er3+ are used as dopants in BGO. BPO crystals are mixed electron-ionic conductors (up to 500 degrees C, t(i)=0.14 +/- 0.01). The ionic conductivity, sigma, and relative static permittivity, epsilon(s), are slightly anisotropic, their highest values, sigma=6 exp (-0.64 eV/kT) and epsilon(s)=50 +/-1, are found along the growth direction [(1) over bar 01]. BGO crystals have a low conductivity (1.5 10(-6) S/cm, at 600 degrees C) of a dominant ionic character (t(i)=0.83 +/- 0.02). Doping with rare earth ions decreases the conductivity significantly. The static relative permittivity of both "pure" and doped BGO is equal to 16.4 +/- 0.1. The thermal conductivity and microhardness of BGO are equal to 4.5 +/- 0.1 W/Km or 5.5 +/- 0.1 kN/mm(2), respectively. A slight impurity hardening takes place. (c) 2008 Published by Elsevier B.V.