Microfluidics have seen a steady expansion of the operation toolbox over the last decade, which includes membrane separations as well. However, the latter are mainly limited to low-pressure operations such as dialysis, MF and UF with only very few reports focusing on high-pressure processes such as NF and RO. In this report a simple high-pressure microfluidic cell suitable for accommodating NF and RO membranes is described and critical design points are discussed. It is shown, both theoretically, using computational fluid dynamics (CFD) and Leveque correlation, and experimentally, that a smaller height of the feed channel is beneficial for minimizing concentration polarization. Minimization of overall pressure losses, hydraulic and osmotic, indicates an optimal channel height of about 40-50 mu m. The NF/RO microcell was tested as a concentrator for solution of a model peptide. The solution was successfully concentrated, however, a significant loss of peptide was observed, presumably, due to adsorption on the membrane or cell walls. This problem will need to be addressed in future studies of NF/RO microcells, however, this work demonstrates the potential and feasibility of implementing RO and NF operations in microfluidic technology. (C) 2012 Elsevier B. V. All rights reserved.