Substitution of ethyl glycol by neopentylglycol in ester-imide oligomers increases solubility and decreases glass transition temperature, while increase of quantity of dimethylterephthalate has opposite effects. Oligomer containing neopentylglycol and the lower quantity of dimethylterephthalate, which dissolves even in mixtures of methyldipropyleneglycol/xylene or toluene 1/1 by weight to obtain 40% or more concentrated solution, is the most convenient. Heat resistance, mechanical and electrical properties, and dielectric losses are optimum if oligomers are cured by heating for 4 h at 260 degrees C. Curing consists in increase of molecular weight and transformation of oligomers in polymers by the elimination of water and diols. Thermal degradation of polymers involves three stages : elimination ol unreacted comonomers and of a part of bound diols; elimination of the rest of diols, dimethylterephthalate and tris-(2-hydroxvethyl)isocianurate; elimination of structures trimellitic anhydride - diaminodiphenylmethane containing aromatic and imidic rings. Neopentylglycol and large quantities of dimethylterephthalate reduce the heat resistance of polyester-imides.