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The anionic polymerization of tert-butyl methacrylate (TBMA) in tetrahydrofuran, using Na+ and Cs+ as counterions proceeds without side reactions even at room temperature. The resulting molecular weight distributions are nearly monodisperse (Mw/Mn ⩽ 1,01). The rate constants for the propagation of ion pairs were measured in the range from + 15 to −100°C. The Arrhenius plots are linear, but different for the two counterions, resulting in the following numerical values for the frequency exponent A and the activation energy Ea: A = 8,5 and Ea = 7,2 kcal/mol = 30 kJ/mol for Na+; A = 9,5 and Ea = 5,6 kcal/mol = 23 kJ/mol for Cs+. The difference between the counterions, which is in contrast to the polymerization of methyl methacrylate (MMA), as well as the lower rate constants are not due to the steric factor of the tert-butyl group but to its positive inductive effect which influences the structure of the propagating macroion pair by decreasing its reactivity. The relative reactivities of the PMMA and PTBMA ion pairs towards MMA and TBMA are estimated.