Aqueous solutions of guar galactomannan and hydroxypropyl guars (HPG) with different molar substitution (MS) levels were studied using dilute solution viscometry and gel, the added hydroxypropyl groups sterically block the hydrogen bonding sites on the guar backbone and reduce the hydrogen bonding attractions between guar molecules. The effects of molar substitution on the intermolecular interactions are inferred from measurements of the Huggins coefficients, which measure intermolecular interactions in dilute solution, and molecular volumes, which reflect intrachain associations. The behavior can be divided into three regimes: (1) at low MS levels (0 < MS < similar to0.4), there is a sharp decrease in intermolecular interactions as a function of MS; (2) in the intermediate range (similar to0.4 < MS < similar to1.0), interactions become independent of MS; (3) at high substitution levels (MS > similar to1.0), the temperature dependence of inter- and intramolecular hydrophobic interactions produces a temperature dependence in the Huggins coefficient and molecular volumes that is not seen at lower substitutions. By acid hydrolysis, HPG samples with a range of molecular weights and consistent polydispersities were obtained. On the basis of these samples, the Mark-Houwink-Sakurada parameters and "characteristic ratio" C-infinity were evaluated for HPG (MS similar to0.6) and compared to the values for guar. The HPG chain stiffens as the degree of substitution increases.