With recent advances in the direct Floating Catalyst CVD synthesis and its on-going commercialisation [1,2], the significant quantities of ‘kilometre’-long CNT fibres could reach the market in the nearest future. This expands their applicability in cost-effective light-weight engineering composites with enhanced toughness and damage tolerance [3,4]. During the composite processing, the internal (inter-bundle) nanoscale pores of a CNT fibre undergo infiltration by a monomer that could be subsequently cross-linked or polymerised. The thin polymer interphase emerging within this hierarchical structure is particularly important for better stress transfer between neighbouring CNT bundles, with significant impact on the overall mechanical performance of a composite. In this work, we present the first-of-this-kind analysis of the chemical structure of CNT fibre-reinforced nanocomposites by means of AFM-IR spectroscopy [5]. The morphology and polymer distribution within the infiltrated CNT fibre could be unravelled with sub-40 nm spatial resolution, including nanoscale chemical imaging of the polymer interphase formed along each single CNT bundle within the nanocomposite. In addition, the complementary mapping of local stiffness could be reconstructed across the area with CNT localization. The results and established methodology reveal new possibilities to analyse reinforcing potential of CNT fibres and intricate CNT-polymer nanocomposites. Acknowledgements NUS Strategic Funding (R-265-000-523-646), malopolska Regional Operational Programme (MRPO.05.01.00-12-013/15), and NUS Materials Science group for providing CNT fibres. References: [1] T. Gspann, F. Smail, A. Windle, Farad Discuss, 2014, 173:47-65. [2] C. Hoecker, F. Smail, M. Pick [et al.], Chem Eng J, 2017, 314:388-395. [3] J. Vilatela, R. Khare, A. Windle, Carbon, 2012, 50:1227-1234. [4] A. Mikhalchan, T. Gspann, A. Windle, J Mater Sci, 2016, 51(22):10005-10025. [5] A. Dazzi, C. Prater, Chem Rev, 2016.