Thin-film chromel-alumel multijunction thermal converters with a low output resistance of 64-85 S2 showed approximately the square law-dependent input-output relation. The voltage responsivities were very low with 0.34∼0.67 V/W in air and 1.15∼1.48 V/W in vacuum, respectively, and the ac-do voltage transfer error was very large with about +340 ppm in the frequency range of 40 Hz∼10 KHz in the case of 1 V-input sinewave rms voltage. It can be concluded that the large transfer error of the thermal converter was mainly caused by the low voltage responsivity and the large heat loss due to low output resistance, which implies that the optimization for small ac-do transfer error is required.