Abstract Knowledge about supercritical heat transfer to refrigerants with horizontal flow is scarce and experimental data is mostly lacking. This makes an accurate sizing of heat exchangers impossible. In addition, the majority of the tested geometries in literature are limited to an inner diameter of only 16 mm. In order to close the gap and increase the knowledge about supercritical heat transfer, a novel experimental setup was used to investigate supercritical heat transfer phenomena on larger tube diameters. The test section consists of a horizontal counter current tube-in-tube heat exchanger. R125 flows in the inner tube having an inner diameter of 24.77 mm. The influence of mass flux (226 - 585 kg/m2s), heat flux (7.7 - 21.9 kW/m2) and supercritical pressure (3.7 - 4.1 MPa, corresponding to 1.03 - 1.12 times the critical pressure) on local forced convective heat transfer was investigated. No visible influence of pressure on the convective heat transfer coefficient is noticed, indicating that buoyancy is present and influential in the performed measurements. The applied heat flux does have an effect, with a decrease in the heat transfer coefficient for an increase in heat flux. This effect is more pronounced at higher mass fluxes. At high mass flux, increasing the heat flux with 42% results in a drop in heat transfer coefficient of 46%, while this is only 40% and 26% at medium and low mass flux, respectively. However, there appears to be a limit to this detrimental effect at higher mass fluxes. Finally, heat transfer increases for an increase in mass flux. Increasing the mass flux with 76% results on average in a rise in heat transfer coefficient of 88%, for measurements at medium heat flux. At high heat fluxes, an increase of 71% is observed for a rise in mass flux of 59%. Nine heat transfer correlations are evaluated. The majority underestimates the heat transfer significantly, however, two correlations operate adequately and are proposed as a basis for future correlation development. The unique dataset and insights presented in this work enable a better understanding of the heat transfer in supercritical vapor generators. Future work includes incorporating top and bottom wall temperature measurements, adjusting control and application of the heat flux and investigating refrigerants with a low Global Warming Potential.