Near-field infrared microspectroscopy (NFIR) is a newly developed surface analysis method that is based on functional-group analysis and has a high spatial resolution. The objective of the present study is to perform nanoscale functional-group analysis of dialysis membrane surfaces by using NFIR. We focused on polyvinylpyrrolidone (PVP), which is employed as an additive to hydrophilize and create pores in synthetic polymer dialysis membranes, and evaluated the PVP distribution on the inner surface of the dialysis membranes. Dialysis membranes made from polysulfone (PSf) and polyester polymer alloy (PEPA) were first assessed by NFIR and then by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). The nanoscale NFIR analysis showed heterogeneous distribution of PVP on the PSf membrane even though the amount of PVP on the membrane surface was high. PVP was homogeneously distributed on the PEPA membrane even though the amount of PVP on the membrane surface was low. In contrast, the microscale ATR-FTIR results showed that PVP was homogeneously distributed on both PSf and PEPA membranes. PVP-rich and PVP-poor regions were distinguishable by NFIR and not by ATR-FTIR, because the spatial resolution of NFIR is higher than that of ATR-FTIR. This study demonstrates for the first time that NFIR can provide nanoscale chemical information on the structures of porous membranes. (C) 2010 Elsevier B.V. All rights reserved.