Packing free energies and structural transitions of concentrated arrays of guar galacto-mannan macromolecules and of guars modified by hydroxypropyl substitution (HPG) have been studied using the osmotic stress method combined with X-ray scattering. All show a liquid crystalline structure with packing free energies that are very similar for guar and HPG and well described by the model of Selinger and Bruinsma for entropic steric repulsion between chains. In addition, a transition from the liquid crystalline form to a crystalline structure is observed as native guar becomes more densely packed. This transition is related to the propensity of guar to form intermolecular hydrogen bonds in solutions. Hydroxypropyl substitution of galactomannan hydroxyl groups causes steric interference that decreases the stability of this hydrogen-bonded crystalline structure. Even for moderately hydroxypropyl-substituted guar (similar to0.3 HP/sugar residue), the transition occurs at a much higher osmotic pressure than for native guar. The extra work needed to crystallize this HPG compared with guar is calculated to be 3 kT/mannose unit or 6-7 kT per hydroxypropyl group. No transition was found for more highly substituted guars. Urea increased the osmotic pressure necessary for the transition of guar but also resulted in new crystalline packing structure.