This work details a technique for the in situ preparation of alumina nanoparticles in heavy oil and explores their activity with respect to mild thermal cracking. In principle, in situ-prepared nanoparticles display a high level of dispersion, which should improve their catalytic activity. Dispersed alumina nanoparticles 17 +/- 5 nm in mean diameter were successfully prepared at 300 degrees C and characterized using X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The thermal cracking experiments were conducted in a batch reactor setup under two stages of heating at 300 and 350 degrees C. The pressure buildup in the reactor and the viscosity and API gravity of the resultant oil were taken as measures of the extent of thermal cracking. Although there was a general shift toward higher degrees API gravity, it still fell within the level of uncertainty probably due to agglomeration at 350 degrees C that limited nanoparticle activity. A higher viscosity was obtained for the liquid fraction because of cross-linking. Furthermore, the uptake of hydrocarbon by the in situ-prepared nanoparticles was compared with that of commercial alumina nanoparticles. Uptake values with and without n-heptane or dichloromethane washing suggest different adsorbed species on the two types of particles.