Mainly used for heat transfer applications, nanofluids are defined as a fluidcontaining nanoparticles. The fluid will serve mainly as solvent, being generally wateror oil. Among the existing nanomaterials, carbon nanomaterials present outstandingproperties, especially their thermal conductivity which has been the subject of studyfor researchers mainly in recent years. Using carbon nanomaterials in nanofluidsoffers the possibility to exploit these amazing properties by bringing them to amacroscopic context1,2. To obtain a high performance nanofluid, a proper dispersionof the nanomaterial in the solvent is mandatory. Unfortunately, carbon nanomaterialsurface is highly hydrophobic with a high tendency to self-aggregate due to theexistence of strong van der Waalsforces which hold them to properly disperse inmost polar liquids3. Therefore, a chemical modification or functionalization of thematerial surface is compulsory. This last usually involves the use of strong oxidantsthat will help to increase the hydrophilicity by attaching functional groups on thematerial surface. However this modification disrupts the nanoparticlestructuredirectly hindering its native properties, being the thermal conductivity not anexception.In this work high performance carbon-based nanofluids were prepared usingfunctionalized carbonnanotubes and graphene. With the aim to clarify the questionsthat are important and that still remain open since they are delicate to deal with, weevaluated the strength and impact of different functionalization treatments in thenanofluid final thermal conductivity. A wide study of the obtained nanofluids stabilityand thermal conductivity was also performed. At first, both nanotubes and graphenewere functionalized by 3 different covalent methods. Later, the functionalization wasconfirmed with the help of electron microscopy, Raman spectroscopy and X-rayphotoelectron spectroscopy as well as by mass spectrometry coupled withthermogravimetry analysis. Secondly, the already modified nanoparticles served toprepare nanofluids. Nanofluid thermal conductivity and stability were later evaluated.For each nanofluid,the thermal performances and the stability over time of theprepared nanofluids were discussed by taking into account the functionalizationstrength applied to each kind of carbon nanomaterials.