Abstract:At  the ultracold neutron(UCN) source of the TRIGA research reactori n Mainz,we have measured for the first time the material optical wall-potential of cubic boron nitride. The measurements were performed with a time-of-flight (TOF) spectrometer. The samples investigated had a wall-potential of ð305715Þ neV. This value is in good agreement with the result extracted from neutron reflectometry data and theoretical expectations. Because of its high critical velocity for UCN and its good dielectric characteristics, cubic boron nitride coatings (isotopicallyenriched will be useful for a numberof applications in UCN experiments.