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Phase change material nano-emulsions are investigated as potential working fluids for various applications including heat transfer and solar applications. Main hindrances to their application are the difficulty in maintaining emulsion stability and the subcooling effect. In this study, new phase change material nano-emulsions (PCMEs) were designed and characterized as possible heat transfer media to be used at 21 and 55 °C, by investigating two commercial phase change materials melting at these temperatures. In particular, the influence of graphene oxide (GO) and reduced graphene oxide (rGO) on emulsion stability and on thermal and optical properties of PCMEs has been investigated. The stability of emulsions was verified and no phase separation or significant growth of PCM droplets were observed through storage and after freeze–thaw cycles. GO was less successful in case of paraffin melting at 21 °C, while it was effective in reducing subcooling to only 1 °C for nanoemulsion with paraffin melting at 55 °C and strongly influenced heat transfer capability. This resulted in a fluid thermal capacity increase of more than 20% within 10 °C from PCM melting temperature. The partial reduction of GO did not result in significantly changing the thermal properties of nanoemulsions and slightly increased the optical absorption, due to higher optical absorption of rGO with respect to GO. [Display omitted] [ABSTRACT FROM AUTHOR]