The advantages of the hetero-junction CdS/ CuAl 2 O 4 for the photocatalytic eosin degradation are reported. Composite semiconductors are elaborated by coprecipitation of CdS on the spinel CuAl 2 O 4 giving a coreshell structure with a uniform dispersion and intimate contact of the spinel nanoparticles inside the hexagonal CdS. The Mott-Schottky plots (C-2-V) of both materials show linear behaviors from which flat band potentials are determined. The photoactivity increases with increasing the mass of the sensitizer CdS and the best performance is achieved on CdS/CuAl 2 O 4 (85 %/15 %). The pH has a strong influence on the degradation and the photoactivity peaks at pH 7.78. The dark adsorption eosin is weak (*4 %), hence the change in the eosin concentration is attributed to the photocatalytic process. The degradation follows a zero-order kinetic with a rate constant of 5.2 9 10-8 mol L-1 mn-1 while that of the photolysis is seven times lower (0.75 9 10-8 mol L-1 mn-1).