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A study of aging processes of polyarylates at low degrees of conversion revealed that crosslinking predominates during both thermal and thermooxidative processes. To evaluate the degree of crosslinking during the aging of polyarylates, calculation of the density of the polymer network has been carried out for the insoluble fraction of polymers according to Flory. An increase in temperature results in a regular increase of cross-linkages. Polymer Ph-2, however, loses the ability to swell at a greater rate than D-9. The effect of oxygen on the amount of crosslinking is insignificant. The molecular weight has been found to increase in the temperature range of 250–275°C., this phenomenon is a result of branching as consequence of degradation processes. An increase of the temperature to 300–350°C. results in formation of insoluble structures. At 250–350°C. both crosslinking and cleavage of the macromolecule chain occur. Formation of three-dimensional structures has also been proved by calculation of crosslinking and cleavage probabilities. Polyarylate Ph-2 undergoes crosslinking to a higher degree. This is due to the peculiarity of its composition, in that its side substituent contains the easily broken lactone ring. The presence of this substituent increases the probability of crosslinking; this is well confirmed by other data obtained by us.