The line-breadth parameters of a number of lines of the $X$-band magnetic resonance spectrum of ${\mathrm{O}}_2$ 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 ($\frac{\Delta \nu }{P}$) is found to be $T^{-0.75\mathrm{}}$. This may be compared with the theoretical $T^{-0.627\mathrm{}}$ and the $T^{-0.85\mathrm{}}$ found by previous workers. We find ${\mathrm{O}}_2$-${\mathrm{N}}_2$ collisions to be no less effective than ${\mathrm{O}}_2$-${\mathrm{O}}_2$ collisions in producing broadening.

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