Thermal, mechanical, turbidity, and microscope evidence is provided which strongly suggests molecular interpenetrating network (IPN) formation by mixtures of the seaweed polysaccharides agarose and kappa-Carrageenan. Over a range of ionic strength, and potassium content, there is no evidence for synergistic coupling of the networks, and simple phase separation (demixing) can definitely be ruled out. At low ionic strength, where the agarose gels first, differential scanning calorimetry evidence shows some influence of the carrageenan on the agarose ordering enthalpy, particularly at higher polymer concentrations. As the potassium level is increased, however, and the order of gelling is reversed, this effect disappears. Cure behavior for the systems at high ionic strength can be described as a simple summation of the pure component contributions. At low ionic strength, on the other hand, the modulus behavior is more complex, suggesting either a modification, in the mixture, of the kappa-carrageenan gelling parameters or a more complex modulus additivity rule.