The use of 1,4-dioxane or pyridine as solvents for the polymerization of isoprene mediated by the acid-functional SG1-based alkoxyamines N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-O-(2-carboxylprop-2-yl)hydroxylamine (BlocBuilder MA) and N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-O-(2-carboxyleth-2-yl)hydroxylamine results in an increase in the rate of consumption of the initiator and narrower molecular weight distributions of the resulting polymer. In pyridine, an improved control of the polymerization was also obtained for a non-acid-functional initiator, and the overall rate of polymerization increased. These effects are likely to be the result of the disruption of intramolecular hydrogen bonding and, in the case of pyridine, stabilization of the polar SG1 free radical.