The three LnOF compounds (Ln = Tm, Yb, Lu) have been prepared and shown to be isostructural with the alpha-ZrO2, baddeleyite. The crystal structure has been refined by a Rietveld analysis of the X-ray powder pattern of the YbOF compound (space groupe P2(1)/c, a = 5.5052(4) angstrom, b = 5.4245(4) angstrom, c = 5.5219(4) angstrom, beta = 99-degrees-68, Z = 4). The strict anionic order observed (alternate 4(4) layers of oxide anions and 3(2).4.3.4 layers of fluoride anions) has been analyzed with respect to the parent fluorite structure and to other isostructural compounds like alpha-ZrO2, TaON.