In this work, we report the use of modified zeolite-clay composite ultrafiltration membranes (preparation of the unmodified (Z(1)), the nitrated (Z(2), modified using NOx) and the aminated (Z(3)) membrane have been described in our earlier publication [1]) for the separation of chromic acid solution. Experiments show that the observed rejection coefficients of the membranes increase on modification from similar to 20% for Z(1) to similar to 40% for Z(2) and similar to 50% for Z(3) membrane. Experimental data is analyzed using the irreversible thermodynamic approach (Spiegler-Kedem equations) [2]. The study shows that the chemical modification of membranes leads to a rise in the intrinsic rejection (highest R similar to 50% for Z(1) to similar to 60% for Z(2) and similar to 66% for Z(3) membrane) coefficients. It is also found that while the observed rejection show anomalous trend with variation in pressure while the intrinsic rejection increases with increase in pressure, a trend typical of the separation of electrolyte through charged membranes. (c) 2006 Elsevier B.V. All rights reserved.