This study provides the first direct experimental measurements of the off-diagonal relaxation matrix element coefficients for line mixingin air-broadened methane spectra for any vibrational band and the first off diagonal relaxation matrix elements associated with line mix-ing for pure methane in the m2 + m3 band of 12CH4. The speed-dependent Voigt profile with line mixing is used with a multispectrumnonlinear least squares curve fitting technique to retrieve the various line parameters from 11 self-broadened and 10 air-broadened spec-tra simultaneously. The room temperature spectra analyzed in this work are recorded at 0.011 cm1 resolution with the McMath-PierceFourier transform spectrometer located at the National Solar Observatory, Kitt Peak, Arizona. The off-diagonal relaxation matrix ele-ment coefficients of m2 + m3 transitions between 4410 and 4629 cm1 are reported for eighteen pairs with upper state J values between 2and 11. The observed line mixing coefficients for self broadening vary from 0.0019 to 0.0390 cm1 atm1 at 296 K. The measured linemixing coefficients for air broadening vary from 0.0005 to 0.0205 cm1 atm1 at 296 K. 2007 Elsevier Inc. All rights reserved.