Summary: The mineral chernovite, described as a new mineral from the Urals by Russian mineralogists in 1967 (Goldin et al, 1967), has been found at three different localities in mineral fissures in the Binnatal, Switzerland, and a little to the south, in Italy. The mineral occurs in greenish-yellow bipyramidal crystals up to about I mm. A detailed single-crystal study of several specimens showed that the lattice constants vary considerably on either side of those published for the original chernovite. The refractive indices, too, display some variation. Microprobe analyses of this material, and of xenotime specimens from the same region, revealed that there exists at least partial solution between chernovite and xenotime. The three chernovite samples have molar percentages of 63·2, 73·2, and 82·0 % YtAsO4 (the Urals chernovite has about 95\2 %). These studies prove the existence of an isomorphous series between xenotime and chernovite, at least in the As-rich portion.