This work aims to understand the formation routes of a variety of interstellar molecules. The formation of molecules in the interstellar medium (ISM) is a key process to understanding the chemical diversity of the universe as well as the origins of biological life on Earth. Some of the molecules (methyl formate and glycolaldehyde) under study have been identified in the ISM via telescopic detec- tion. Other molecules investigated have been proposed to be present in the ISM as a result of terrestrial laboratory studies on simulated ISM ices (urea, ribose and benzoic diacids). Quantum chemical calculations were used to follow the re- action pathways of these molecules and test their feasibility under astrochemical conditions. It was found that the presence of hydrogen bonding ices around the reaction species greatly affects the energetics of the reactions and increases their likelihood to occur in the ISM. Additionally, water ice was found to have a large im- pact on the dissociation of methanol into radical species allowing for much more facile subsequent reactions than previously thought. Water ice was also found to affect the ease of ionisation of polycyclic aromatic hydrocarbon (PAH) species which have been proposed as a constituent of ISM dust grains.