Due to their atomically excellent structure and sp2-hybridization, carbon nanotubes (CNTs) have exceptional mechanical, thermal, and electrical properties. Single-walled CNTs (swCNTs) have recently gained interest as an intriguing host for the nanoscale confinement of fluids, with a variety of unexpected phenomena such as spontaneous filling, frictionless mass transport, unusual phase diagram etc[1]. The majority of these phenomena are still under debate and require experimental confirmation. However, it is challenging in this field to find experimental methods sensitive enough to carry out measurements at the level of individual nanotubes. The electronic properties of swCNTs are demonstrated to be sensitive to their diameter, defects, doping, adsorbates, and environment via swCNT field effect transistors (swCNT-FETs)[2], [3]. In this contribution, we show that individual carbon nanotube field effect transistors (CNTFET) are the perfect tool for achieving this goal, enabling for the first time to accurate identification of water confined inside the nanotube. By investigating the electrical performances of several unopened and opened CNTFETs submitted to different atmosphere and temperature treatments, such as dry air, humidity, secondary vacuum, and current annealing, we demonstrate that it is possible to distinguish between water being outside and inside of the nanotube, just outside, or the nanotube free from water. Hence, we found that secondary vacuum and current annealing both shift the threshold gate voltage of opened swCNTs towards more negative values, however, the secondary vacuum had no effect on closed swCNTs compared to current annealing. In conclusion, the current annealing process is essential to distinguish water adsorbed outside from water confined inside swCNT. We demonstrate that this behavior is uniform across all devices’ metallicities, assuming that the nanotube’s surface has been pre-cleaned using current annealing. We will also discuss the mechanism behind the coupling of electronic transport and the presence of water. Our findings open up the possibility of using CNTFET to address long-standing issues in the nanofluidic community regarding the behavior of water confinement at the nanoscale, and also to use them for reliable, sensitive, and selective chemical and biological sensors. Reference: [1] T. A. Pascal, W. A. Goddard, and Y. Jung, “Entropy and the driving force for the filling of carbon nanotubes with water,” Proc. Natl. Acad. Sci., vol. 108, no. 29, pp. 11794–11798, 2011, doi: 10.1073/pnas.1108073108. [2] D. Cao et al., “Electronic sensitivity of carbon nanotubes to internal water wetting,” ACS Nano, vol. 5, no. 4, pp. 3113–3119, 2011, DOI: 10.1021/nn200251z. [3] I. Heller, A. M. Janssens, J. Männik, E. D. Minot, S. G. Lemay, and C. Dekker, “Identifying the mechanism of biosensing with carbon nanotube transistors,” Nano Lett., vol. 8, no. 2, pp. 591–595, 2008, DOI: 10.1021/nl072996i.