Both linear and branched poly(ethylenimine) (LPEI and BPEI), when partially neutralized with H3PO4, behave as solid proton conductors under dry conditions. Conductivity at a fixed temperature depends strongly on H3PO4/LPEI ratio, x. It first increases to a maximum as x is increased from 0 to ca. 0.1, then experiences a 10(2) fold decrease to a minimum at x = ca. 0.4, where the LPEI has been protonated to its limit and the dominating anion is HPO42-. Additional H3PO4 beyond this point introduces H2PO4-, and conductivity again increases with x (eg, 1.0 x 10(-5) S cm(-1) for x = 0.8 at 60 degrees C); the hydrogen bond arrays of the coexisting H,PO, and HPO:- are presumably responsible. Our interpretation is practically the same as that by Lassegues et al., but the 10(3)-fold improvement in conductivity with BPEI-H3PO4 over LPEI-H3PO4 that they had claimed was not substantiated by our experiments.