Abstract The cross section of the reactions 3He(d, p) 4 He and d(3He, p)4He has been measured at the center-of-mass energies E=5 to 60 keV and 10 to 40 keV, respectively. The experiments were performed to determine the magnitude of the electron screening effect leading to the respective electron-screening potential energy U e =219±7 and 109±9 eV, which are both significantly higher than the respective values from atomic physics models, U e =120 and 65 eV.