The nanosize effects of NiO nanosorbcats on adsorption and post-adsorption catalytic thermo-oxidative decomposition of vacuum residue (VR) n-C-5 asphaltenes was investigated using a UV-vis spectrophotometer and thermogravimetric analyzer coupled with a mass spectrometer. Sizes between 5 and 80nm of different-sized NiO nanosorbcats were employed. Batch adsorption experiments were carried out for the considered asphaltenes in toluene solutions, monitored via UV-vis spectrophotometry. The macroscopic adsorption isotherms were described by implementing the solid-liquid equilibrium (SLE) model. The findings showed that thermally cracked vacuum residue (VR) n-C-5 asphaltenes interact to different extents with different-sized NiO nanosorbcats. A normalized surface area basis was used for the amount of VR n-C-5 asphaltene adsorbed per nm(2) of NiO surface, which was the highest for NiO nanoparticles of size 80nm, with 5nm size being the lowest. Thermogravimetric analysis of VR n-C-5 asphaltenes was also achieved and the reaction products were explored by a mass spectrometer. The Kissinger-Akahira-Sunose (KAS) isoconversional model was used to describe the reaction mechanism and to confirm the validity of the catalytic role of the different particle sizes of NiO nanosorbcats. The highest catalytic activity was for smallest NiO when compared to the highest NiO nanosorbcats. Furthermore, the results of thermodynamic transition state parameters of activation; changes in Gibbs free energy (Delta G(double dagger)), entropy (Delta S-double dagger), and enthalpy (Delta H-double dagger) highlighted the catalytic activity of NiO nanosorbcats towards VR n-C-5 asphaltenes oxidation. These findings exhibit the significance of textural properties and nanosize of nanoparticles during adsorption and thermal catalytic processing of asphaltenes.