Supercritical (sc) fluids are extensively used for material processing and carbon dioxide is the most commonly used solvent. In these solutions, solute-solvent interactions consist primarily of Van der Waals forces and the small amount of data published shows that the solute has only a small effect on solution structure. The use of more polar solvents allows high concentrations of polar solutes to be dissolved resulting in more ordered solutions. This has an effect on clustering and mass transport. The aim of this thesis is to investigate solute-solute-solvent interactions in sc difluoromethane and to quantify the effect on solution viscosity for the first time. The local composition about the solvatochromic probe has been measured as a function of pressure and solute polarity and the Kamlet-Taft polarisability/dipolarity and hydrogen bond donor parameters determined. Variations in these solvent properties are understood in terms of solute aggregation and a model for solvation is proposed. The use of a piezoelectric quartz crystal as a reliable high-pressure viscometer is proposed and used to determine the viscosity of sc solutions as a function of pressure and solute polarity. A modification of the Dole-Jones equation is used to model the viscosity of simple sc solutions and the volume fraction of solute is shown to be the key factor affecting solution viscosity. Rapid expansion of supercritical solutions (RESS) is a technique used to reduce particle size and alter morphology of pharmaceuticals. As with most sc processes carbon dioxide has been the solvent of choice, but many pharmaceuticals are polar compounds or salts, which suffer from low solubility. Here the applicability of sc difluoromethane for precipitation of materials by RESS is presented.