The upper liquid-liquid transition (T l ) 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 liquidliquid 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 l 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 l temperature was lower in a polyester containing a diole with more flexible structure. The changes in electrical conductivity and 1 H NMR spectra indicated that the breaking of hydrogen bonding caused the T l . 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 l , 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.