The upper liquid-liquid transition (T-lp) was detected and investigated in unsaturated polyesters. This less known transition is caused by a stepwise decrease of intramolecular short-range local order that remains above the glass and lower liquid-liquid transitions. The local order is based on secondary valent interactions and is enhanced by hydrogen bonding, if possible, as in the polyesters under consideration. The T-lp was detected as a change in the temperature dependence of the viscosity and electrical conductivity and differential scanning calorimetry thermograms revealed an endothermic change, above which the activation energy of crosslinking decreased. The T-lp temperature was lower in a polyester containing a diole with more flexible structure. The changes in electrical conductivity and H-1 NMR spectra indicated that the breaking of hydrogen bonding caused the T-lp. Its intramolecular nature resulted in low sensitivity to crosslinking and addition of solvents. Near-IR spectroscopy was chosen for further investigation because of its greater sensitivity to structural changes in polymers than mid-IR. The spectra were recorded at selected temperatures. Significant intensity changes and wavelength shifts of hydroxyl and carbonyl absorptions at 1430 and between 1900 and 2100 nm, respectively, occurred at the T-lp, while carbon-hydrogen absorptions showed only minor changes. This confirmed that breaking of the hydrogen bonding was responsible for disruption of the short-range local order. (C) 2000 John Wiley & Sons, Inc.