Summary: Electron diffraction and electron-microprobe investigations have revealed the presence of order in the monazite-huttonite series. Chemical analyses from the literature and electron-microprobe analyses in this study show that the monazite-huttonite series, REPO₄-ThSiO₄, is only continuous if the contribution of F, M²⁺, and OH to the charge balance is taken into account. The electron-diffraction study points to ordered domain structures in the series.