Results of an investigation are presented which was carried out on the influence of the structure of the initial reagents, of the content of the crosslinking monomer in the resins ((copolymers), and also of the crosslink density of the copolymers, on their resistance to the action of water, acids, alkalis and moist chlorine gas at various temperatures. Several commercial Soviet high chemical resistant grades of polyester were tested. These polyester resins are characterized by relatively low water absorption, and retain their strength in full when held at room temperature in water for 1 to 1. 5 years.