Static and dynamic light scattering, viscosity, and optical rotation measurements have been made at eight different temperatures between 25 and 75 degreesC on two succinoglycan samples (sodium salt) with weight-average molecular weights M-w of 7.14 x 10(5) and 3.54 x 10(5) (at 25 degreesC) in 0.01 M aqueous NaCl to investigate the thermally induced order-disorder conformation change of the polysaccharide. Additionally, viscometry and polarimetry have been performed for a sodium salt sample (Mw = 4.55 x 10(5) at 25 degreesC) whose M-w, z-average radius of gyration (1/2)(z), and hydrodynamic radius R-H in the aqueous salt had been determined previously. As the temperature increases, M-w, (1/2)(z), R-H, and the intrinsic viscosity for every sample sharply decrease around 55 degreesC where the specific rotation at 300 nm sigmoidally increases. In particular, Mw at 25 degreesC (i.e., in the ordered helical state) is twice as large as that at 75 degreesC (i.e., in the disordered state). These findings substantiate that the ordered structure is composed of two chains and hence is a double helix. Data analysis shows that this helix at 25 degreesC is characterized by an unperturbed wormlike chain with a helix pitch of about 2 nm (per repeating unit) and a persistence length of about 50 nm and that upon heating, it dissociates directly (i.e., in all-or-none fashion) to disordered chains of a similar contour length but with a much smaller persistence length of about 10 nm. The temperature dependence of the light scattering second virial coefficient is discussed in relation to the association of disordered chains in the cooling process.