We have measured the absorption profiles of CH3F mixed with argon and helium at room temperature in the interval 940-1110 cm(-1) of the nu(3) band. The spectra measured at pressures of 0.25-3 bar were used for evaluation of the line-broadening parameters. At higher pressures up to 20 bar significant deviations from the Lorentz bandshape were detected. At the maximum of the Q branch the absorption is super-Lorentzian, the differences reaching up to 60%. In the lower frequency wing of the P branch the observed absorption is up to five times lower than the Lorentzian one. The discrepancies observed are sensitive to the perturbing molecule, being more pronounced in the case of helium. They are explained by line-mixing effects and interpreted by means of a simple model with a single adjustable parameter. (C) 1997 Elsevier Science Ltd.