Hydrogen atom isomerisations within five radical systems (i.e., CH₃˙NH/˙CH₂NH; CH₃O˙/˙CH₂OH; ˙CH₂SH/CH₃S˙; CH₃CO₂˙/˙CH₂CO₂H; and HOCH₂CH₂O˙/HO˙CHCH₂OH) have been studied via quantum-mechanical hydrogen tunnelling through reaction barriers. The reaction rates including hydrogen tunnelling effects have been calculated for these gas phase reactions at temperatures from 300 K to 0 K using Wenzel–Kramers–Brillouin (WKB) and Eckart methods. The Eckart method has been found to be unsatisfactory for the last two systems listed above, because it significantly underestimates the width of the reaction barriers for the interconversions. The calculations at all-electron CCSD(T)/CBS level of theory indicate that the barriers for all reactions (forward and reverse) are greater than 100 kJ mol⁻¹, meaning that the chemical reactivity of the reactants is limited in the absence of hydrogen tunnelling. Hydrogen tunnelling, in some cases, enhance rates of reaction by more than 100 orders of magnitude at low temperature, and around 2 orders of magnitude at room temperature, compared to results obtained from canonical variational transition state theory. Tunnelling corrected reaction rates suggest that some of these isomerisation reactions may occur in interstellar media.