Investigations of the structure of indole, cocaine hydrochloride, alprazolam and clonidine hydrochloride in solution were undertaken using a combination of experimental, principally neutron di?raction, and computational techniques. This thesis aims to probe the interactions of the hydrophilic and hydrophobic groups of each these molecules with their solvent environment, drawing conclusions about how their interaction with solvent molecules and their conformation in solution are related to their biological function. Cocaine, alprazolam and clonidine are of particular interest because of their ability to cross the blood-brain barrier. Investigation of the solvation of indole, the functional group of the amino acid tryptophan, in methanol/water solutions highlighted the role played by electrostatic interactions between the benzene ring of indole and its environment in dictating the location of tryptophan in the membrane bilayer. By studying the hydration of cocaine in aqueous solution, a possible mechanism explaining the ability of cocaine to cross the blood-brain barrier in its protonated form, which involves shielding of its hydrophilic groups through a water mediated internal hydrogen bond, was proposed. The preferentially solvated regions of the benzodiazepine alprazolam in methanol/water solutions indicated that these regions coincide with the location of bridging water molecules in the crystal structure of alprazolam bound to the BRD4 protein, which regulates transcription of oncogenes. A similar analysis of the solvation of clonidine in methanol/water solutions supported hypotheses relating to the mechanism of action of clonidine occurring via binding to adrenoceptors rather than imidazoline receptors. The work presented in this thesis demonstrates the importance of determining the structure of biomolecules in solution in gaining a comprehensive understanding of their behaviour in vivo.