During membrane process design, flat sheet test cell experiments are performed to determine intrinsic membrane properties. Since they can be easily performed in the laboratory and combine easy sample replacement with the use of a small membrane area, they are clearly preferable to module experiments. However, the small active membrane area used in such test units may cause inconsistent results due to the membranes being inhomogeneous. Estimation of membrane material heterogeneity is an important step to increase the reliability of test cells application to membrane process design, but investigations of the possibility of increasing the uniformity of membrane properties are essential. We found inhomogeneity being partially caused by improper pre-treatment of the membrane material. In this paper, investigations of conditioning methods for NF and RO membranes prior to initial use are presented. It should be mentioned that the aim is not a complex modification of membrane material, but finding a simple technique providing a better membrane wetting. Simply flushing with clean water, as recommended by the manufacturers, appears to be insufficient. Results obtained by applying three different methods: flushing with commercial membrane cleaner, with alcohol in water solution and treatment with high pressure are discussed in detail.