Stress relaxation and swelling measurements were carried out and the data used for determination of the effective crosslink densities of one- and two-component urethane sealants. These crosslink densities were compared with calculated values. The results were correlated with the structure of the urethane sealants. Two sealant structures were evaluated: one based on mixtures of triol prepolymers and diol adducts, and the other on triol adducts. Both one- and two-component sealants were investigated. The two-component sealants had a tendency to be undercured even when quite effective catalysts were used. The one-component sealants, because of moisture cure and of biuret or allophanate formation, produced in most cases a higher crosslink density than the calculated triol crosslink density. It was also observed that curing efficiencies of the two-component sealants decreased with increasing molecular weight per crosslink. Deviations from these correlations were noted when a short poly(oxypropylene)glycol of 400 molecular weight was used for curing purposes. These anomalies were detected by stress relaxation and by swelling techniques as well as by measurement of torsional modulus. Stress relaxation studies were carried out to determine the influence of temperature, catalysts, and strain on stress decay. The results again indicated that the two-component sealants did not cure completely, and that the type of catalyst had a significant influence on degradation of urethane sealants at elevated temperatures.