To investigate their ignition delay and combustion behavior, experiments with two biomass pyrolysis oils and No. 2 diesel fuel were performed in a direct injection diesel engine. It was found that while the indicated thermal efficiency of both pyrolysis oils equaled that of the diesel fuel, they exhibited excessive ignition delays and required a moderate degree of combustion air preheating to ignite reliably. Despite the longer ignition delays associated with the pyrolysis oils, the cylinder pressure rise rates were significantly less than with No. 2 diesel fuel. Experimental ignition delay and heat release rates were interpreted using a phenomenological spray combustion model. Using a three parameter fit for vaporization, ignition, and combustion rate, the model showed that the longer ignition delays of the bio-oils result from slow chemistry relative to diesel fuel. The model also showed that the heat release profiles of the bio-oils are consistent with slow combustion chemistry and rapid mixing relative to diesel fuel. As a result, whereas diesel combustion is predominantly mixing limited, pyrolysis oil combustion is predominantly limited by chemistry through much of the process.