Experimental studies have noted the often surprising and unpredictable effect of ionic liquids as solvents on reaction kinetics for radical polymerisation. We theoretically investigate the energetic and structural effects of ionic liquids, both protic and aprotic, on radical stability, presenting stabilisation of the radical by the ionic liquid by up to −78.0 kJ mol−1. Kinetic data relating to propagating systems for several industrially viable monomers indicate that propagation rates can be increased or decreased (by up to 6 orders of magnitude) depending on the monomer and ionic liquid combination. The interplay of activation entropy and activation enthalpy, much of which depends on hydrogen bonding between the solvent and reactants, play a crucial role in controlling reaction kinetics. It is concluded that the use of cheaper protic ionic liquids as solvents may be viable for improved kinetic control over radical reactions.