The propagation rate constant for ion pairs (k(p)(+/-)) in the polymerization of isobutylene in conjunction with TiCl4 in hexanes/methyl chloride 60/40 (v/v) at -80 degreesC has been determined using two different diffusion clock methods. The rate constant k(p)(+/-) was in the range of (0.3-1.0) x 10(9) L mol(-1) s(-1) 4 orders of magnitude higher than presently accepted values. The first method involved on-line UV-vis monitoring of the addition of the pi -nucleophiles 1,1-bis(4-methylphenyl)ethylene, 1,1-bis(4-tert-butylphenyl)ethylene, and 2-phenylfuran to hydrochlorinated isobutylene n-mers (n = 2, 3, 36). The apparent rate constants of capping, k(0)K(i), and the rate constant of ionization, k(i), have been determined. For a given n the k(0)K(i) values were identical independent of the nature and nucleophilicity of the pi -nucleophile, which was attributed to diffusion-limited addition. Using the diffusion-limited second-order rate constant of he similar to 3 x 10(9) L mol(-1) s(-1), K-i and k(-i) have been calculated. From the concentration of active chain ends (determined from K-i) and the apparent rate constant of propagation for isobutylene (k(p)(+/-)[active chain ends], determined separately), the absolute propagation rate constant of k(p)(+/-) = 1 x 10(9) L mol(-1) s(-1) was calculated. The second simple diffusion clock method involved competition experiments, i.e., polymerization carried out in the presence of a pi -nucleophile, which stops short of completion when all chain ends are capped. From the limiting conversions and number-average degrees of polymerization, k(p)(+/-) = (3-6) x 10(8) L mol(-1) s(-1) have been obtained.