Modulation broadening of the iodine absorption components of a 127I2 stabilized He-Ne laser at the transition 11-5, R(127) is studied in this report. The mathematical model used for computer simulation is described. Simulation results are presented for the frequently used d, e, f and g components and compared with the results of the measurements. The effect of 127I129I components on 127I2 absorption spectra is investigated. The level of the relative 127I129I line strength can be readily found by fitting the simulation results into the measured frequency shift data. The limitations of the conventional matrix analysis in the presence of 129I are considered.