The ternary compound Er2Mn2/3Mo4/3O7, with a pyrochlore-related structure, was synthesized in a vacuum-sealed quartz tube at 1373 K, The compound was characterized by X-ray diffractometry, electron probe microanalysis, and transmission electron microscopy. The compound possessed a monoclinic structure, with lattice parameters of a = 1.2781(2) nn, b = 0.7378(5) nm, c = 1.1643(6) nm, and beta = 100.53(1)degrees. The compound had a large number of lamellar domains, which were composed of a microtwinned structure on the twin axis <(1) over bar(1) over bar0 >.