The reaction of gadolinium with fluorine was studied in a low pressure, transonic flow reactor. Atomic fluorine reactant was generated by microwave discharge dissociation of F₂ in argon. Kinetic measurements included the rate of change of the electrical resistance of isothermal gadolinium filaments due to reaction with atomic and molecular fluorine and the intensity of chemiluminescence from the vapor phase Gd/F₂ reactionvs time, temperature, and F₂ partial pressure. The surfaces of reacted Gd specimens were examined by scanning electron microscopy, polished specimen cross sections were viewed under the optical microscope, and the Gd/F₂ chemiluminescent emission spectrum was obtained. A nonpassivating, metal rich, high melting and involatile fluoride, GdF_x, forms on the metal at temperatures above 1180 K. An orthorhombic, passivating GdF₃ forms at lower temperatures. The surface Gd activity remains high at the GdF_x/fluorine interface, and F-atom reaction is first order with a reaction probability approximately 0.28. Rapid reaction atT < 1180 K is followed by an abrupt transition to passivated behavior. This change in reactivity is attributed to a phase transition from GdF_x to orthorhombic GdF₃ that occurs as the oxygen impurity concentration of the reacting metal decreases from its value near the original specimen surface to that of the bulk material.