The pyrolysis of cyclopropane + argon mixtures has been studied in a single-pulse shock tube over the temperature range 1060–1870 K. At a total pressure of 500 mmHg and temperatures below 1300 K, an apparent first-order rate constant of 1011.9 exp (–230 kJ/RT) s–1 was obtained. This is in agreement with extrapolations from low temperature data provided allowance is made for the deviation from limiting first-order behaviour with increasing temperature. Above 1300 K, radical chain decomposition of the cyclopropane and/or the propylene product was observed but the overall loss of cyclopropane, which is best fitted by the expression 104.75 exp(–48.5 kJ/RT) s–1, was much lower than predicted by current unimolecular reaction rate theory. Various alternative mechanisms are examined but no acceptable explanation can yet be offered.