The effect of thermal history on gel-sol transition was investigated by highly sensitive differential scanning calorimetry (DSC) in order to clarify the non-equilibrium state of kappa-carrageenan hydrogels. kappa-Carrageenan with a concentration from 0.5% to 5.0% was used. When concentration of solution was lower than 2.0%, homogeneous kappa-carrageenan gel was formed when aqueous solution was fully equilibrated. When concentration exceeded 3.0%, a sub-peak could be observed at the low temperature side of the main peak. It was indicated that helices having various sizes and different kinds of defects are present in the junction zone. Thermal histories, such as cooling rate from the sol state or annealing at around gel-sol transition temperature, markedly affect the junction zone formation. A large junction zone is formed at a slow gelation rate, and also that many small junction zones form at a fast gelation speed. (C) 2006 Elsevier B.V. All rights reserved.