Some calorimetric data of the order-disorder conformational transitions of natural polysaccharides from microorganisms are presented in the more general thermodynamic framework of the cooperative helix-coil transition of linear biopolymers. A new home-made data acquisition and processing procedure is presented and discussed. The phase transition is analysed for a neutral capsular polysaccharide from Rhizobium trifolii TA1 and for two ionic exopolysaccharides, xanthan from Xanthomonas campestris and succinoglycan from a strain of Rhizobia. Structural features, such as the linear charge density, are discussed as determining factors in the thermal stability of the helical ionic polysaccharides.