The directly fired supercritical carbon dioxide (sCO₂) power cycle has high efficiency while allowing nearly complete carbon dioxide (CO₂) capture. The operating condition of sCO₂ power cycle (10 MPa to 30 MPa) combustors is dramatically different from conventional gas turbine combustors. However, combustion properties, e.g., autoigntion delays, have never been reported under sCO₂ conditions. An urgent question to be answered for supercritical carbon dioxide oxy-combustion is how well existing chemical kinetic models perform as no experimental data exists at relevant conditions. In this talk, autoignition delays at sCO₂ condition are reported for CH₄/O₂/ and H₂/CO/O₂ mixtures in CO₂ and Ar diluents above their critical pressures (approximately 100 bar). Experiments reveal that a widely used kinetic model, GRI 3.0, underpredicts the ignition delay by a factor of 3 for CH₄. However, kinetic models Aramco 2.0, USC Mech II and HP-Mech are capable of predicting autoignition delays though not validated at these conditions. For H2/CO mixture, all the tested kinetic models could reasonably predict the autoigition delays at supercritical conditions. Detailed kinetic analysis is conducted. Underlying kinetic processes controlling ignition and the effect of CO₂ as diluent is revealed for different fuels.