Magnesium aluminum layered double hydroxide (MgAl-LDH) was first successfully deposited in situ on eta-Al2O3 applied to catalyze urea hydrolysis. Characterization by AAS, XRD, SEM, EDS, DSC, FT-IR, N-2-BET, determination of base strength and basicity and experiments on hydrolysis activities of in situ MgAl-LDH catalysts show that the crystallinity and coverage degree of MgAl-LDH on eta-Al2O3 gradually increase with increasing initial c(Mg2+). When initial c(Mg2+) = 2 mol L-1, eta-Al2O3 is completely covered and the smallest particle size, the highest crystallinity, basicity and urea degradation rate of in situ MgAl-LDH are obtained. The basicity is mainly from Mg-OH at the edges of MgAl-LDH on eta-Al2O3. The highest basicity corresponds to the highest urea degradation rate, Mg-OH is the active sites for catalytic hydrolysis of urea. The specific surface area, basicity and catalytic activity of MgAl-LDH by in situ synthesis are evidently higher than MgO and MgAl-LDH purchased. The urea concentration is 8.162 mg L-1 at 165 A degrees C after 120 min with in situ MgAl-LDH (c(Mg2+) = 2 mol L-1) as catalyst, which is below the effluent standard. In situ MgAl-LDH also shows excellent recycling ability and the slight loss of activity should be due to the reduction of basicity, which is caused by hydrothermal conditions. In addition, a possible catalytic mechanism of urea hydrolysis over MgAl-LDH is proposed. [GRAPHICS] .