Mixed ionic-electronic conducting pyrochlore structure oxides, with Pr and Gd on the A site and Zr, Mn, Ce, Sn, In, Mo, and Ti on the B site, were characterised by X-ray powder diffraction and Raman spectroscopy. Mn and In have a solubility around x = 0.1-0.2 in Pr2Zr2-xMnxO7 and Pr2Sn2-xInxO7, respectively. In the series Pr2M2-xMxO7, where M = Sn, Zr and M' = In, Ce, we observe dopant-O-6 symmetrical stretch vibrations in addition to the host lattice modes. A defect model of a B site doped pyrochlore is developed with Pr3+ on the A site; Zr-B(x) (Zr4+), Ce-B' (Ce3+), Ce-B(x) (Ce4-) on the B site; O-O(x) and V-O on the O site, interstitial oxygens O-i", and delocalised electrons and electron holes. Four mass action law expressions govern such a model. The defect model can rationalise why home-valent doping, i.e. substitution of Zr(4+) by Ce(4+), can lead to an increase in ionic conductivity. The calculated Brouwer diagram for Pr2Zr1.6Ce0.4O7+/-delta is shown. This composition has a transition from mixed ionic p-type to presumably pure ionic conduction around pO(2) = 10(-7.5) atm. At pO(2) < 10(-15) atm the material gradually changes into the n-type regime. Typical magnitudes are finally given for the four equilibrium constants, leading to cases of pure p-type, p- to n-type and pure electrolytic behaviour of doped pyrochlores. (C) 2000 Elsevier Science B.V. All rights reserved.