Advanced hybrid nanofluids have been engineered through the synthesis of aqueous suspensions containing graphene sheets and iron oxide (Fe3O4) nanoparticles at a graphene-Fe3O4 mass ratio of 70:30. Karaya gum and cocoamidopropyl betaine (CAPB) were selected as surfactants given their good affinity with dispersed nanomaterials. Graphene sheets with Fe3O4 nanoparticles as well as the prepared dispersions were first characterized in terms of morphology, chemical composition and stability using various techniques, including electron microscopy, UV–visible spectroscopy and Zeta potential analyses. Given the crucial role that surface tension (ST) plays in boiling/condensation processes or micro-flow applications, this physical property was investigated at nanoadditive mass concentrations of 0.005–0.100 % and temperatures of 283.15–313.15 K using the pendant drop method. As an entry parameter for ST evaluation, the density of these hybrid nanofluids was also measured at the same temperature and concentration ranges. Experimental studies revealed that ST of water reduces with the addition of surfactant and increasing temperature. Graphene-Fe3O4 nanoparticles were also found to further reduce the ST (thus enhancing the effect of the surfactant mixture) by up to 42.7 % on average at 0.1 %wt. compared to water in the tested temperature range. This may be potentially interesting for improving boiling heat transfer performance and optimizing bubble flow pattern.