Three rotational isomeric state (RIS) models for pullulan, a polysaccharide defined as [--> 6)-alpha -D-glucopyranosyl-(1 --> 4)-alpha -D-glucopyranosyl-(1 --> 4)-alpha -D-giucopyranosyl-(1 -->](n), are presented. These RIS models are based upon simple molecular mechanics and solvation calculations and differ in the complexity of the energy evaluations used to determine Boltzmann weighting factors for each isomeric state. Each model is evaluated for its ability to predict correctly experimental small angle X-ray scattering data and the radius of gyration (R-g) for the hexameric pullulan oligomer (G(3))(2). A simple model based upon only disaccharide conformational energies in conjunction with a correction for solvation energies, as determined using a continuum Poisson-Boltzmann solvation method, was best and gave an excellent fit to experimental measurements.