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 °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 °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 °API gravity, it still fell within the level of uncertainty probably due to agglomeration at 350 °C that limited nanoparticle activity. A higher viscosity was obtained for the liquid fraction bec...