Thermal phase transitions of K-carrageenan in pure water and in KCl solution were studied using photon transmission technique. Photon transmission intensity, I-tr, was monitored against temperature to determine the critical coil-to-helix (T-ch) and helix-to-coil (T-hc) temperatures and exponents, beta. T-ch, and T-hc values were found to be higher for the gels containing cations (K+) than for those without cations (K+). The increase in carrageenan content caused an increase in critical temperatures for the gels prepared in pure water and in KCl solution. Increases in the KCl/carrageenan ratio, raised both T-ch and T-hc values. On the other hand, the critical exponents, beta were found to be independent of the system, they are measured by indicating that they fall into the same universality class. (C) 2004 Elsevier B.V. All rights reserved.