Rapid compression machine (RCM) has been employed to study auto-ignitions of methane-air mixtures at intermediate- to high temperatures. The facility allows performing measurements at temperatures of 900−1500 K and at pressures varying from 0.38 to 2.23 MPa. The reaction evolution was measured from pressure and visible emission records. Ignition delay time correlations were generated from the experimental data sets. The Arrhenius temperature approximation of ignition delay times showed a significant reduction of the average activation energy from 53 kcal/mol to 15.3 kcal/mol, at temperatures below 1300 K. Moreover, RCM data showed a big scattering which was increased dramatically with temperature decreasing. The reasons of such anomalous behavior were studied by high-speed visualization techniques and experiments with oxygen free and fuel free mixtures. These tests showed the unavoidable presence of contaminating ultrafine particles in the test volume as well as their capability to auto-ignite and to provoke premature ignition of gas mixtures.