The adverse effect of salts, especially inorganic salts, on electrospray ionization mass spectrometry (ESI-MS) is one of the most serious obstacles that might limit its application. Among the numerous desalting approaches, the microdialysis technique is favorable for large molecules, such as proteins. In this work, employing a hollow fiber membrane of cellulose acetate (MWCO 3000 Da), a simple, facile and efficient microdialysis interface with the dead volume of less than 1 µL was constructed for the on-line desalting and identification of proteins dissolved in high salt concentration buffer by nano-ESI-MS. Furthermore, with counterflow added, the desalting procedure was accelerated, and could be finished within 1 min. This system was successfully applied to the analysis of myoglobin dissolved in either high concentration ammonium acetate or sodium chloride buffer. The experimental results showed that, by using such a microdialysis interface, the salt concentration, even as high as 1 M, could be decreased by at least 2 orders of magnitude, while sample loss was less than 10%, demonstrating the potential of such an interface in broadening the application of nano-ESI-MS in the analysis of large molecules. Copyright © 2008 John Wiley & Sons, Ltd.