The present paper deals with AM mode locking of multiatmosphere CO2 lasers. We measure the effective linewidth in the laser mixtures and deduce from these data the pulse width. The method is based on determining the maximum frequency shift from the synchronized frequency. The advantage of this method is that the results are not influenced by time constant of detector and display. The results were verified by direct measurements of the pulse widths in mixtures up till three atmospheres. The pulse widths and effective line widths of the P(20) transition are determined for the usual CO2 laser mixture up till 6 atm. We observe that helium has a large effect on the pulse width. The width decreases with increasing helium percentage at all pressures investigated. We suggest that this has to do with the overlap of a sequence-band transition and that the effect of sequence bands depend ion the helium percentage.