Dielectric and viscosity techniques were used to determine the degree of the compatibility of poly(methyl methacrylate)/polycarbonate, poly(methyl methacrylate)/polystyrene, and polycarbonate/polystyrene blends in different ratios (25/75, 50/50, and 75/25 w/w). The effect of the addition of 5, 10, and 20% concentrations of the prepared polyester resin [poly(butylene terephthalate adipate)] on the compatibility of these blends was studied. The dielectric properties were measured over a frequency range (from 100 Hz to 100 kHz) at various temperatures covering the glass-transition temperatures of the polymers used (from 30 to 170 degrees C). It was found from the dielectric and viscosity measurements that the addition of 10% polyester to poly(methyl methacrylate) /polycarbonate, 20% polyester to poly(methyl methacrylate) /polystyrene, and 5% polyester to polycarbonate/polystyrene blends enhanced the degree of compatibility of such blends. (c) 2008 Wiley Periodicals, Lnc.