An advanced isoconversional method has been applied to determine the effective activation energies (E) for the glass transition in polystyrene (PS), poly(ethylene terephthalate) (PET), and boron oxide (B2O3). The values of E decrease from 280 to 120 kJ . mol(-1) in PS, from 1270 to 550 kJ mol(-1) in PET, and from 290 to 200 kJ mol(-1) in B2O3. It is suggested that a significant variation in E should be observed for the fragile glasses that typically include polymers.