FT30 type thin film composite membranes used for reverse osmosis water purification are very difficult to analyze. With the remarkably thin polyamide layer and surface modification of the polymers now being reported, a new analytical technique is needed to determine the surface structure and chemical distribution in the active layer. In this study we show that scanning transmission X-ray microscopy (STXM) can be used to determine the spatial distribution of polyamide and polysulfone and we report on the detection of an unexpected new phase. The new phase was identified as a homopolymer of the meta-phenylene diamine (MPD) that forms in MPD solutions and can be incorporated into the discrimination layer during the interfacial reaction with the TMC to produce a mixed polyamide polyMPD layer. The detection of this second phase was only made possible by STXM. At the levels detected in membranes in this study (less than 8%) the second phase had no effect on the flux or salt passage of the membranes, however at higher levels a change in the membrane properties most likely would occur and the quantity of the polyMPD present should be eliminated or controlled. (C) 2011 Elsevier Ltd. All rights reserved.