The reaction between hydrogen atoms and isobutene has been investigated under conditions where [H][i-C4H8] in the temperature range 20–328 °C at an argon pressure of 6.6 Torr in a discharge-flow apparatus. Two ratios of rate constants have been determined, log (k2/k1)= 0.47 ± 0.33–16.6 ± 2.6 kj mol–1/(2.3RT), and k5/k6= 3.55 ± 0.24, independent of temperature. (Error limits are 95% confidence limits.) H + i-C4H8→ t-C4H*9(1), H + i-C4H8→ CH3C3H6(2), H + t-C4H9→ H2+i-C4H8(5), H + t-C4H9→ i-C4H*10(10). The stoichometry of the overall reaction is discussed.