An optical biosensor with a stirred cuvette has been used to monitor the interaction of immobilized wheat germ agglutinin (WGA) with two water-soluble cationic porphyrins. The association constants (Ka) of the free base porphyrin and its Zn(II) complex form were 2.66 and 27.31×105 L/mol at 20°C respectively. The interactions of the free base porphyrin were further investigated at temperatures between 15°C and 37°C. The thermodynamics parameters, changes in free energy, enthalpy and entropy, were −31.23, 22.92, 54.15 kJ/mol respectively. The heat capacity change was −355.53 J·mol−1·K−1. The binding was driven by entropic contribution, and showed strong enthalpy-entropy compensation. It was governed primarily by hydrophobic forces.