This dataset includes dual nitrate stable isotope measurements collected at 11 stations of the R/V Sonne cruise SO305/2 E-POLIO-T & M2Argo-T (16 July – 05 August 2024) along the route between Singapore and Port Louis. This data was used to study nitrogen cycling in the Equatorial Indian Ocean. Samples were collected at 11 stations at selected water depths from free flow bottles attached to the CTD. Samples were filtered (GF+SFCA, MINISART, 0.45 µM) on board and stored frozen (-20 °C) in Falcon PE tubes until analysis in the home lab. Dual nitrate stable isotopes were measured during the denitrifier method (Casciotti et al., 2002; Sigman et al., 2001) using a GasBench II coupled with an isotope ratio mass spectrometer (Delta Plus XP, Thermo Fisher Scientific). If the nitrite concentration exceeded 5 %, it was removed prior to analysis using sulfamic acid (Granger and Sigman, 2009) to get nitrate isotope signatures.