The pyrolysis of asphaltenes under hydrothermal environments covering the subcritical and supercritical regions of water was applied, and the influence of the presence of H-donors was surveyed by a reaction kinetics analysis based on the lumping approach. Under hydrothermal environments, the pyrolysis of asphaltenes consisting mainly of condensation to coke and decomposition to maltenes is significantly faster than that under a N-2 environment. The H-donors introduced, decalin or maltenes, may provide nonaromatic H atoms, capping the carbon radicals essential to pyrolysis. Accordingly, the apparent activation energies of the condensation and the decomposition of asphaltenes both increase to varying degrees. The pyrolysis of asphaltenes in the presence of a small quantity of decalin is seriously retarded in subcritical water but recovers rapidly in supercritical water owning to the promoted initiation efficiency at high temperature. Accompanied by a large amount of maltenes, the decomposition to maltenes involved in the original pyrolysis network of asphaltenes can be neglected.