The large electronic isotope effect in the spin coupling of ${}_{}{}^2{}_{}{}^{}\mathrm{II}$-terms, reported by Johnston and Dawson for boron monoxide and for the heads of the O${\mathrm{H}}^2$ bands is confirmed by measurements of the fine line structure in the isotopic OH bands. Both for BO and for OH the observed increase in the doublet spacings, in the molecule with the heavier isotope, agrees closely with the results to be expected from Hill and Van Vleck's theoretical treatment of spin coupling energy, although their equations do not so well reproduce the separate doublets. This isotope effect is zero at the band origins, increases to a maximum (which amounts to 20 ${\mathrm{cm}}^{-1\mathrm{}}$ in the 0″ vibrational level of OH) and approaches zero asymptotically at high rotational quantum numbers. A large electronic isotope effect in $\Lambda$-doubling is also observed for OH. This effect, which is zero at the band origin and increases with increasing rotation, amounts to 17 ${\mathrm{cm}}^{-1\mathrm{}}$ at $K=25\mathrm{}$. It is not correctly accounted for by Van Vleck's equations for $\Lambda$-doubling although ratios of the $\Lambda$-doublets in O${\mathrm{H}}^1$ and O${\mathrm{H}}^2$ are correctly given, for a case $b$ molecule. No noticeable effect was observed on the spacings of the ${}_{}{}^2{}_{}{}^{}\Sigma$-doublets in the excited state of OH.

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