Thermal behavior was examined using DSC and TG for the samples prepared by dissolving LiClO4 in several polyoxyethylene/polyethylenimine (PEO/PEI) blends (7/3–9.5/0.5 by repeat units) along with their lithium ion conductivity. Concerning the matrix polymers, while the pristine PEO and PEI began to thermally decompose at 185 and 230°C, respectively, the (8/2) PEO/PEI blend had lost only 6.5% of its weight before 300°C was reached. Dissolution of LiClO4 made the systems thermally even more robust: [(8/2) PEO/PEI]10–LiClO4 suffered practically no weight loss below 300°C. So far [(8/2) PEO/PEI]10–LiClO4 has been our best-behaving SPE (σ=2.2×10−4 S cm−1 at 77°C and 2.5×10−6 S cm−1 at 30°C): it is less conductive than a previously reported PEO12–LiClO4 by a factor of 2.2 at 77°C, but its conductivity drop due to the matrix crystallization below 65°C is considerably milder and the conductivity at 30°C was ca. 100 times higher than the plain PEO–LiClO4 systems. A significant effect of the residual methanol on the ionic conductivity of our [(8/2) PEO/PEI]10–LiClO4 samples was recognized, which necessitated our previously reported log σ vs. 1000/T curve to be corrected downward by ca. half an order of magnitude.