We measured a series of absorption profiles of the nu(3) band of CH3F perturbed by helium. The measurements were performed at room temperature for helium pressures from 1 to 90 atm. We observe significant deviations of measured profiles with respect to those calculated as a sum of lines having Lorentzian shapes. To interpret the observed deviations, we consider various models of line mixing. The calculations performed in the ECS approximation provide the most reasonable agreement with the measured band shapes reproducing the existing data on line broadening coefficients within the limits of experimental uncertainty. Special attention is paid to the analysis of the differences of interbranch and intrabranch line mixing, that is to the structure of relaxation matrix.