The significance of pyrolysis reactions in the early stages of diesel combustion has received little attention in the literature, which warrants a mechanistic investigation of the controlling chemistry along with its potential impacts on the overall combustion process in engines. Experiments were performed in a constant volume vessel to probe these pyrolytic reactions, where diesel fuel sprays were injected into air at varied pressures and temperatures chosen to represent an engine operating at various loads. The pressure inside the vessel was found to decrease immediately following the start of injection before increasing as the exothermic heat release occurs. The initial pressure decrease has been conventionally attributed to an evaporative cooling effect of the diesel spray, but the objective of this paper is to test the hypothesis that endothermic pyrolysis reactions can make a significant contribution to the observed pressure decrease. The addition of 1% of the cetane booster, 2-ethylhexylnitrate (2-EHN), to the fuel was found to shorten the measured ignition delay time, as expected. However the presence of 2-EHN can also increase the magnitude of the initial pressure decrease compared to conventional diesel fuel. Detailed chemical kinetic modeling shows that the effects observed in the constant volume vessel can plausibly be attributed to pyrolysis reactions, and that the addition of 2-EHN to the base fuel enhances their influence. The modelling results also imply that the influence of these pyrolysis reactions increases with increasing temperature, pressure, the alkyl chain length of the base fuel, and the amount of any radical initiator in the fuel.