In this work a detailed X-ray powder diffraction study of the reversible temperature transformation has been undertaken in several rare earth niobates MNbO4 M being lanthanum, cerium, neodymium, gadolinium, holmium, thulium, ytterbium, and yttrium. Both high-temperature diffractometer and film techniques were used. It was shown for the substances investigated that the low-high transformation is a continuous transition over the wbole temperature range from monoclinic at room temperature to tetragonal at the transition temperature. It was found that the rate of the overall transition is rapid, and also that the transition between any intermediate state occurs reversibly within minutes or seconds. The continuous transition and the rapid rate of attainment of equilibrium in the solid, even between intermediate states of the transformation, support the earlier assumption that it is a displacive-type order-disorder transition.