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This study examines how non-covalent and covalent surface modification methods affect the stability and thermo-physical characteristics of carbon nanotubes (CNTs) and graphene platelets (GNP) when distributed in solar thermal fluids. Surfactants are employed for examining non-covalent surface modification, while acid treatment is utilized for investigating covalent modification. Nanofluid samples are created with concentrations of 0.0625%, 0.125%, 0.25%, and 0.5% by weight in pure ethylene glycol (EG) and EG–water (80:20). Zeta potential analysis was utilized to study the stability of the nanofluids. The oxidized CNT–GNP nanofluids showed outstanding stability, with zeta potential values remaining almost constant for 60 d. Experiments were conducted on pure, surfactant-dispersed, and oxidized CNT–GNP nanofluids. The thermal conductivity of oxidized CNT–GNP nanofluids increased by 16% when 0.5 wt% was dispersed in the EG–water (80:20) sample. Nanofluids with CTAB and SDS showed a 14% and 13% enhancement in thermal conductivity, respectively. Formulas were created to forecast thermal conductivity and dynamic viscosity values.