The main focus of this dissertation was on characterizing the viscoelastic behavior of a set of four flexible slabstock water-blown polyurethane foams with varying hard segment content as well as solid plaques made from these foams. Three viscoelastic tests; tensile stress relaxation, compression load relaxation, and compression creep, were utilized to evaluate the behavior of these materials at constant temperature and/or relative humidity, RH. The tensile stress relaxation tests were performed at a 25 percent strain level. The majority of the compression load relaxation tests were conducted at a 65 percent level since this is the strain level used for the common indentation load deflection test for flexible foams and the relaxation behavior was rather independent of strain at this level. Over a three hour testing period, a near linear relationship for the log of tensile stress or compressive load versus log time is observed for most conditions. The slope from this linear relationship in tension or the stress decay rate is similar for all the foams and their respective plaques; thus indicating that the tensile stress relaxation of these materials is governed by the solid portion of the foams and is therefore independent of the cellular textures. In addition, the rates of relaxation for rather linear behavior in tension and compression are also comparable for these foams and this implies that the relaxation in compression is mostly independent of the cellular texture of the foams at a 65 percent strain level. After a short induction period, the compressive creep behavior exhibits rather linear behavior for linear strain vs log time over a three hour period. The slope of this relationship is dependent on the initial strain level and goes through a maximum with initial strain at 40 percent. This maximum is believed to be due to the buckling of the foam’s struts. The results for the creep behavior were evaluated at a 65 percent initial strain since the creep behavior is believed to be mostly independent of the cellular texture of the foam at this level and greater. A greater amount of viscoelastic decay, i.e. tensile stress relaxation, compression load relaxation and com- pression creep is observed for the higher hard segment foams. Temperature has a similar effect on the results obtained from the three viscoelastic tests. Likewise, relative humidity at a constant temperature also has a similar effect on the viscoelastic behavior of the three tests. Up to 100°C, temperature accelerates the viscoelastic decay of these foams over a three hour time period. For all three viscoelastic tests, a significant increase in the viscoelastic decay at temperatures greater than 100°C is observed. The FTIR thermal analysis of the plaques indicated that this significant increase is due to additional hydrogen bond disruption as well as possible degradation in the urea and urethane links. Increasing relative humidity at a given temperature does bring about a steady decrease in the initial load or initial stress as well as a small increase in the rate of viscoelastic decay. Overall, the effects of temperature are greater on the viscoelastic decay than humidity. The morphology and the viscoelastic behavior of another set of flexible slabstock foams were characterized. These additional foams are rather unique in that some of their morphological features, the urea aggregate structure in particularly, are altered by adding a small amount of LiCl to the formulation. As discussed within the body of this dissertation, these observed changes in morphology are believed to have a significant effect on the viscoelastic nature. / Ph. D.