Both linear and branched poly(ethylenimine) (LPEI and BPEI), when partially neutralized with H 3 PO 4 , behave as solid proton conductors under dry conditions. Conductivity at a fixed temperature depends strongly on H 3 PO 4 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 HPO 2 - 4 . Additional H 3 PO 4 beyond this point introduces H 2 PO - 4 , and conductivity again increases with x (eg, 1.0 10 - 5 S cm - 1 for x = 0.8 at 60°C); the hydrogen bond arrays of the coexisting H 2 PO - 4 and HPO 2 - 4 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-H 3 PO 4 over LPEI-H 3 PO 4 that they had claimed was not substantiated by our experiments.