Urea-formaldehyde (UF) resins are susceptible to stress rupture and hydrolytic degradation, particularly under cyclic moisture or warm, humid conditions. Modification of UF resins with flexible di- and trifunctional amines reduces this problem. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the thermal behavior of modified and unmodified adhesives to identify the physical and morphological factors responsible for the improved performance. A UF resin modified by incorporating urea-capped poly(propyleneoxidetriamine) during resin synthesis exhibited a higher cure rate and greater cure exotherm than the unmodified resin. Resins cured with a hexamethylenediamine hydrochloride curing agent had slower cure rates than those cured with NH4Cl. DMA behavior indicated that modified adhesives were more fully cured and had a more homogeneous crosslink density than unmodified adhesives. DMA behavior changed with storage of specimens at 23 degrees C and 50% relative humidity, after previous heating for approximately 20 min at 105 degrees C to 110 degrees C. The initial changes were postulated to occur because of physical aging (increase in density) and continued cure. These were followed by physical breakdown (microcracking) and possibly cure reversion.