AbstractThe ionisation (Ar2CHCl + BCl3⇌ Ar2CH+BCl−4) and dissociation (Ar2CH+BCl−4⇌ Ar2CH++ BCl−4) equilibria of diarylmethyl chlorides in boron trichloride/dichloromethane solution have been studied by conductimetry, photometry and1H NMR spectroscopy. Small differences in the UV‐vis spectra of diaryl‐carbenium tetrachloroborates, which have been observed in solutions of low and high tetrachloroborate concentration, can be attributed to the formation of 1:1 ion‐pairs in the more concentrated solutions. Low temperature calorimetry was used to determine the heats of addition of diarylcarbenium tetrachloroborates to 2‐methyl‐1‐pentene (Ar2CH+BCl−4+ H2C‐CRR' + Ar2CH‐CH2‐CRR'Cl + BCl3), and it is estimated that the standard free enthalpy of this reaction is greater than 0 for systems with pKR+>−2.6. Kinetic studies have shown that paired and unpaired diarylcarbenium tetrachloroborates exhibit identical reactivity towards alkenes. A rationalisation for the different situation in carbocationic and carbanionic polymerisation is presented. The rate constants for the initiation of isobutene, styrene and isoprene polymerisation by diarylcarbenium ions have been determined, and it is shown, how propagation rate constants can be estimated from l