In this study, the viscoelastic and morphological properties of molded foams were investigated to determine the influence of the presence or absence of reinforcing particulate copolymer polyols (CPP). The molded foams were based on toluene diisocyanate (TDI) and glycerol-initiated ethylene-oxide endcapped polypropylene oxide and, in most samples, some amount of copolymer polyol. Two series of foams were studied. In Series 1, as CPP is added to the formulation, the amount of TDI fed is kept constant. This results in a constant amount of hard-segment content as the filler in the system displaces, by weight, the polyether polyol in the foam, and it increases the hard segment to soft segment ratio (HS/SS). In Series 2, the amount of hard-segment material is proportionally decreased as CPP is added, resulting in a constant HS/SS ratio. Structural investigations of the foams displayed rather similar textures. The cellular structures of a CPP-containing foam was very similar to a foam lacking the copolymer polyols, Transmission electron microscopy revealed that the CPP particles were well dispersed and that they possessed significant rigidity even at high temperature and under high compression. Although all of the foams were microphase-separated, they varied slightly in that the copolymer polyol containing foams exhibited higher weight fractions of extractables in both Series 1 and Series 2. This suggests that not all of the CPP material is covalently bonded into the polyol matrix. It; was found that temperatures above ambient as well as humidity plasticized the viscoelastic behavior of all the molded foams evaluated. It was also found that the copolymer polyol particles, as added to the molded foams of Series 1, increased load-bearing capabilities but had a negative effect on the stress relaxation, creep, and compression set properties. In particular, the viscoelastic properties of the CPP-containing foam were distinctly more time-dependent than those of the foam lacking these particles. However, the Series 2 foams show that most of these effects are a result of the increased HS/SS ratio and not a result of the CPP particulate. It was shown that adding CPP while maintaining a constant; HS/SS ratio improves percent load loss and load bearing under high-humidity conditions, two important properties in flexible polyurethane foams, Finally, it was shown that at high temperatures (ca. 100 degrees C), an additional relaxation mechanism occurs which cannot be attributed to changes in the HS/SS ratio, but must be a result of the CPP components themselves. This additional mechanism results in higher rates of load relaxation and creep in foams containing CPP at high temperatures for foams of both series.