The photodissociation dynamics of dicyclopropyl ketone are investigated using time-resolved Fourier transform infrared spectroscopy and photofragment ion imaging spectroscopy. The photodissociation products are C3H5+CO+C3H5, and the isomerization dynamics of C3H5 are the focus of this paper. Electronic structure calculations are used to define the potential energy surface, while a two-step phase space theory model predicts excitation in the CO product. The vibrational energy distribution of the CO product is not described by this statistical distribution, and is more excited than that observed in the analogous dissociation of acetone. The translational energy distribution of CO indicates an exit barrier on the potential energy surface. Contrary to expectations based on the photodissociation of other aliphatic ketones, the hydrocarbon products are not cyclopropyl radicals. Instead, the excited dicyclopropyl ketone undergoes a ring-opening isomerization to form diallyl ketone, followed by dissociation producing allyl radicals and carbon monoxide. Some of the allyl radicals have sufficient internal energy to decompose to allene+H. (C) 2003 American Institute of Physics.