Phase transitions and thermal behavior of fuel-diluent mixtures were investigated at temperatures and pressures up to 750 K and 600 bar, respectively, as part of a breakthrough approach directed to mitigate the harmful emissions of diesel fuel combustion and provide simultaneous benefits on the engine efficiency and environmental thermal impact. (Anitescu, G. PhD Thesis, Syracuse University, Syracuse, 2008. Tavlarides, L. L.; Anitescu, G. US Patent No. 7,488,357, 2009.) The mole fraction of fuels (e.g., n-hexadecane or cetane and diesel fuel No. 2) was in the range of 0.100-0.786 in mixtures with chemically inert diluents such as CO2 and mixtures of CO2, H2O, and N-2 as substitutes for exhaust gases of diesel engines. Most of the experiments were conducted in laboratory flow and batch reactors equipped with view cells to visualize the phase transitions associated with the heating processes of the fuel-diluent mixtures. The Soave-Redlich-Kwong equation of state with one-parameter conventional mixing rule was used to construct P-T phase diagrams to identify the conditions for the experiments carried out with no view cell included in the laboratory setups. The extent of thermal decomposition of both cetane and diesel fuel No. 2 was relatively low in the presence of the selected diluents, proving the role of these additives as anticoking agents in the newly proposed diesel engine applications of the heated fuels. The major reaction products of cetane thermolysis, identified by GC-MS analytical technique, were all pairs of C-8-C-14 n-alkanes and homologue alpha-olefins. Only under highest P-T-X conditions were some polycyclic aromatic hydrocarbons and higher molecular alkanes produced in low amounts.