Ni-Mo-Al eutectic alloys were subjected to a process of unidirectional solidification (UDS) by the Bridgman-Stockbarger technique with a view to a composite of ductile matrix containing ductile reinforcement of comparatively high strength. The microstructure after UDS consists of a γ/γ' matrix reinforced with Mo(α) fibers. The growth direction and the orientation relationships between the phases were determined by electron diffraction. In the present study, attention was focussed on phase stability after prolonged exposure at elevated temperature. Isothermal exposure of 90 min at 1250°C was followed by a mild water quench. Apart from the beneficial influence on the γ' morphology, partial transformation of the α(Mo) fibers into δ-NiMo intermetallic was observed. Replacement of the ductile α(Mo) fibers by the brittle δ-NiMo is accompanied by partial loss of facetting. Decisive evidence of this diffusion-controlled phase transformation is supplied by TEM micrographs.