Aromatic nitriles constitute a key component of numerous commercial compounds including pharmaceutical, agrochemicals, pigments and dyes. Benzonitrile derivatives such as Diclobenil (2,6-Diclorobenzonitrle) and bromoxynil have been extensively used as herbicides. Their utility also stems from the numerrous possible nitrile transformation, including the synthesis of benzoic acid, amidines, imidoesters, benzamidines, amines, heterocycles and aldehydes. Many important multi-step synthesis reactions pass through a nitrile intermediate. Examples include the synthesis of ibuprofen and ketoprofen. Among heterogeneous catalysts, MCM-41 molecular sieves have attracted much research attention owing to their high surface area, thermal and hydrothermal stability and ordered mesoporous nature. In the present study, Al-MCM-41 with various SiO 2 /Al 2 O 3 ratio was synthesized and modified by impregnation with phosphoric acid. The catalysts characterized by XRD, BET, FT-IR, Si-MAS-NMR and the acidity of the catalysts were measured by pyridine adsorbed FT-IR spectroscopy. Their catalytic performance was examined in the liquid phase dehydration of cinnamaldoxime in the temperature range of 60-180 ?C. Then we studied the amount of H 3 PO 4 loaded on the surface of catalysts and various SiO 2 /Al 2 O 3 ratio on conversion of oxime and selectivity of cinnamonitrile. The liquid phase dehydration of cinnamaldoxime over H 3 PO 4 /Al-MCM-41 revealed that 30%wt.H 3 PO 4 /Al-MCM-41(70) is more active than the other catalysts. The effect of solvent and acid over the best catalyst were studied and the results are discussed. Finally dehydration of other aldoximes was investigated in the best conditions and the productes characterized by FT-IR spectroscopy. The results reveal that 30%wt.H 3 PO 4 /Al-MCM-41 shows a good selectivity and activity in dehydration of aldoximes.