Long range ordered Cu3Au exhibits like other L12 ordered alloys an anomalous increase in the yield strength with increasing temperature. Dislocations which have an antiphase boundary APB fault on the cube plane play a main role in theoretical models that have been proposed to explain this anomaly. In this investigation two types of loops are studied that occur on cube planes. From the measured dissociaton widths of the superlattice dislocations the value of the APB energy can be determined. Different values have been achieved for octahedral and cube planes; they can be compared with previous measurements and with the theoretical models.Single crystals of Cu3Au were grown under vacuum and subsequently annealed to achieve a high degree of order and a large domain size. The crystals were deformed in tension at room temperature. For the TEM investigation discs were cut parallel to the primary octahedral glide plane and to the cube cross slip plane. The preparation of the thin foils is rather difficult because of the large gold content.