The a.c. electrical properties (dielectric constant and loss index) of several series of crosslinked isophthalic polyester resins were investigated over the frequency range 50 to 105 cycles/sec. and over the temperature range −60°C. to above the temperature for maximum loss index. In one series of these resins, the ratio of isophthalic acid to maleic anhydride was varied, in another the chain length of the diol was varied, and in another the concentration of styrene was varied. The temperatures at which the maximum values of the electrical properties occurred are shown to be related to the heat distortion temperature, and within the range of compositions studied, could be predicted from chemical structure. The data suggest that a ratio of isophthalic acid to maleic anhydride of less than 1 : 1, short-chain diols, such as 1,2 propanediol, and more than 50% by weight of styrene will yield the best electrical properties in polyesters of the type studied.