Ferroelec. oxyfluoride K3MoO3F3 single crystals prepd. by the Bridgman method were studied by means of polarized light microscopy and dielec. measurements. Anal. of the ferroelastic domains indicate a first order phase transition at TC = 528 K from a ferroic phase to a cubic (m.hivin.3m) one. Domain structures are consistent with a trigonal symmetry below TC. Continuous measurements of the temp. dependence of birefringence and dielec. permittivity between 300 and 600 K led to the disclosure of an addnl. phase transition of 1st-order at T1 = 453 K.