Hypothesis: The bulk and the surface structure of nanoparticles based on poly(lactic-co-glycolic acid) (PLGA) and PLGA-PEG copolymer is expected to consist of different polymer compositions. An enrichment of hydrophilic PEG on the surface in combination with an accumulation of PLGA in the bulk is anticipated. Hence, the imbalance between bulk and surface composition should be detectable by suitable analytical methods. Experiments: Nanoparticles were assembled using an emulsion-evaporation method with polyvinyl alcohol (PVA) as stabiliser. Mixtures of PLGA and PLGA-PEG copolymer were applied to achieve variably PEGylated nanoparticles. The nanoparticle composition was analysed with respect to PLGA, PVA and PEG, comparing the polymer content of the nanoparticle bulk to the surface. For the bulk, PVA was quantified by a UV/vis spectroscopic method as well as size exclusion chromatography (SEC), and H-1 nuclear magnetic resonance (NMR) spectroscopy. PEG determination of the bulk was carried out using quantitative H-1 NMR spectroscopy. Surface composition was investigated by X-ray photoelectron spectroscopy (XPS). Findings: For the characterisation of the polymer composition H-1 NMR, SEC, and XPS-methods were successfully established and validated. Unexpectedly, a significant PEG content was detected within the particle bulk. The comparison of nanoparticle bulk to surface showed a surface accumulation of hydrophilic PVA with increasing PEGylation. (C) 2015 Elsevier B.V. All rights reserved.