Technical Abstract: Hard-to-boil (HTB) massecuites with markedly low heat transfer properties are a sporadic but continuing problem in Louisiana sugarcane factories. It usually occurs after severely deteriorated sugarcane stalks and trash have been delivered to a factory. Unfortunately, the specific cause of this phenomenon is unknown and only limited correction in the boiling house has occurred. HTB and normal massecuites and molasses were collected from four Louisiana factories in 2006. Compared to normal samples, the HTB samples had 9.1-33.2% lower heat conductivity and 10.0-49.2% higher heat resistivity. The more HTB a sample is, then the greater the increase in heat resistivity compared to the corresponding decrease in heat conductivity. This strongly indicates substance(s) are responsible for suppressing heat transfer. Excess lime addition during juice clarification is not the direct cause of reduced ability to boil. Oscillatory deformation rheology whereby a sinusoidal shear force is applied to a sample and the resultant stress response measured provides information on the mechanical and viscoelastic properties of the material as well as intermolecular network associations. Normal massecuite and molasses samples gave typical mechanical spectra of concentrated solutions. In contrast strong intermolecular (gel) networks were present in the HTB samples explaining the difficulty in removing water on boiling. As HTB samples contained considerably greater total soluble polysaccharides than the normal samples, a polysaccharide is likely the cause of the gel network. Initial results suggested the presence of an arabinogalactan and endo-dextranase resistant dextran structures. The HTB phenomenon may have different causes and mannitol is a contributing but not major factor.