Coil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of t-carra-geenan in various cation (CaCl2) solutions upon heating and cooling were studied using the photon transmission technique. Photon transmission intensity, I-tr, was monitored against temperature to determine the (c-h) and (h-d) transition temperatures (T-ch and T-hd) and activation energies (Delta E-ch and Delta E-hd). An extra dimer-to-dimer (d-d) transition was observed during cooling at low temperature region. However, upon heating dimers directly decompose into double helices by making dimer-to-double helix (d-h) transition. Further heating results in double helix-to-coil (h-c) transition at high temperature region. T-dh and T-hc temperatures and Delta E-dh and Delta E-hc activation energies were determined. It was observed that T-hc and T-ch temperatures and Delta E-ch and Delta E-hc activation energies were found to be strongly correlated to the CaCl2 content in the system.