Supercritical pyrolysis mechanisms of potential endothermic fuels and hydrogen donor additives decahydronaphthalene (decalin) and tetrahydronaphthalene (tetralin) were examined in a specially constructed silica–lined flow reactor in which pressure and temperature were varied independently. Pressure and temperature were varied over ranges of 700—810 K and 0.2 to 10 MPa. Under these conditions, major products of supercritical decalin pyrolysis included: light alkanes and alkenes, methylhexahydroindane, indene, methylcyclohexenes, and indane. Major products of supercritical tetralin pyrolysis included: naphthalene, methylindane, ethane, methane, ethene and phenyl butane. The major products found in these experiments are contrasted with those found in gas phase pyrolysis studies. Quantification of the major products indicated that C6to C5ring contraction was found to occur preferentially with increasing pressure. The formation of these more compact products is qualitatively consistent with a reaction model that incorporates cage effects.The addition of hydrogen donors decalin and tetralin to decane resulted in a slight increase in the thermal stability of decane. A 30% conversion of decane in the decane/H–donor mixtures was observed to occur at approximately 790 K versus 770 K in the neat decane for a fixed residence time and pressure. However, the real potential of these H–donor additives with regard to fuel thermal stability may be in increasing the overall volatility of the products of pyrolysis by promoting the formation of saturated products. Finally, global kinetic parameters are presented for each neat fuel and mixture