The line-breadth parameters of a number of lines of the X-band magnetic resonance spectrum of O2 have been measured and interpreted with the aid of a previously reported theory of the spectrum. The results are that the M-dependence of the line width is negligible, and there is only a slight decrease in width with increasing K. A typical width (half-width at half-intensity) at 300°K is 2.2 (Mc/sec)/mm Hg. At pressures up to 8 mm Hg, the line centers have been found to shift less than 2 percent of the line width. The average temperature dependence of the normalized line breadth parameter (∆νP) is found to be T-0.75. This may be compared with the theoretical T-0.627 and the T-0.85 found by previous workers. We find O2-N2 collisions to be no less effective than O2-O2 collisions in producing broadening.