This paper presents turbulent burning velocities, S-T, of several premixed CH4/diluent/air flames at the same laminar burning velocity S-L = 0.1 m/s for two equivalence ratios phi = 0.7 and 1.4 near flammability limits with consideration of radiation heat losses from small (N-2 diluted) to large (CO2 diluted). Experiments are carried out in a cruciform burner, in which the long vertical vessel is used to provide a downward propagating premixed flame and the large horizontal vessel equipped with a pair of counterrotating fans and perforated plates can be used to generate an intense isotropic turbulence in the central region between the two perforated plates. Turbulent flame speeds are measured by four different arrangements of pairs of ion-probe sensors at different positions from the top to the bottom of the central region in the burner. It is found that the effect of gas velocity on S-T measured in the central region can be neglected. Simultaneous measurements using the pressure transducer and ion-probe sensors show that the pressure rise due to turbulent burning has little influence on S-T. These measurements prove the accuracy of the S-T data. At phi = 0.7, the percentage of [(S-T/S-L)(CO2) -(S-T/S-L)(N2)]/(S-T/S-L)(N2) decreases gradually from -4 to -17% when values of u'/S-L increase from 4 to 46, while at phi = 1.4 such decrease is much more abrupt from -19 to -53% when values of u'/S-L only increase from 4 to 18. The larger the radiation losses, the smaller the values of S-T. This decreasing effect is augmented by increasing u'/S-L and is particularly pronounced for rich CH4 flames. When u'/S-L = 18, lean CO2 and/or N-2-diluted CH4 flames have much higher, 3.6 and/or 1.8 times higher, values Of S-T/S-L than rich CO2 and/or N-2-diluted CH4 flames, respectively. It is found that lean (phi = 0.7) CH4 flames are very difficult to quench globally even when methane mixtures are diluted with 41% CO2 having large radiation losses. Moreover, measurements of chemiluminescence intensities for these turbulent propagating CH4/diluent/air flames using the photomultiplier are carried out. The peak light intensities of CH* and C-2* emitters are found to be qualitatively con-elated with these aforementioned S-T data of lean and rich turbulent CH4/diluent/air propagating flames, respectively. Finally, the scatter plot of S-T/S-L as a function of u'/S-L for the aforementioned N-2- and CO2-diluted CH4 flames can be well compressed and approximated by a general correlation of turbulent burning velocities in a form Of (S-T - S-L)u' = 0.06Da(0.58), where Da is the turbulent Damkohler number. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.