Resolving power, preferred thickness and balance errors of Ross filters for x‐ray monochromatization are discussed. It is theoretically impossible to balance two filters of elementary composition because of scattering,Llimits andKjump, even if the absorption coefficients of the two filters vary similarly with wave‐length between the limits, which is not known to be the case. It suffices, however, to obtain good balance for wave‐lengths near the pass band since others longer and shorter may be relatively reduced by pre‐filtration through a filter whose composition is the same as that of the high atomic number member of the balanced pair. TheKjump balance error may be completely removed by adding a slightly absorbing filter of low atomic number to one member of the pair. By employing a specially designed ionization chamber Ross filters may be used simultaneously instead of alternately and a considerable gain in speed and accuracy achieved.