The effect of toluene diisocyanate (TDI) index on the physical properties, structure, and morphology of flexible slabstock polyurethane foams was investigated. Foams based on a 2700 molecular weight triol, 6 pph water, and varying amounts of an 80/20 mixture of 2,4-and 2,6-TDIs were characterized using a number of physical property and morphological measurements. Extraction experiments using dimethyl formamide (DMF) showed that increasing the index increased the level of covalent crosslinking with perhaps a maximum being reached at an index ca. 100. Viscoelastic measurements also supported the claim of increased crosslinking with TDI index. The initial load in load relaxation experiments at 65% strain systematically increased with increasing TDI while the percent decay in a 3-h period decreased. Temperature and/or humidity ''plasticized'' the load relaxation behavior in all the foams studied, indicating that the hard segment domain physical ''crosslinks'' play a significant role in the properties of these materials. Interestingly, compression set measurements appeared to be independent of the index, likely due to some level of hard segment continuity, but the induced recovery of the compression set at elevated temperatures was indeed sensitive to the index. The amount of recovery systematically increased with increasing TDI index due to the more enhanced ''recoverable'' covalent network. Scanning electron microscopy (SEM) studies of the foams showed that the cellular structure was not significantly affected by the index. However, SEM also showed that the structure of the high index foam was not greatly altered by the extraction process while the lowest index foam's cellular structure was severely disfigured. The fine structure of the foams was found to be influenced by the TDI index. Small angle X-ray scattering, differential scanning calorimetry, and dynamic mechanical analysis all provided evidence that an increase in the TDI index promoted some phase mixing of the soft and hard segments. FTIR showed that the short-range ordering within the hard segment domains displayed a maximum at an index of 100. This was attributed to the concentration of hard segment domains being lower at a lower index and their ordering being disrupted at higher indexes due to more extensive covalent crosslinking prior to completion of phase separation. Wide angle X-ray scattering results also confirmed that for the highest index level, the short-range ordering of the TDI moieties was decreased.