A thin gas target was used to measure the absolute cross-section and angular distribution of α -particles from the two ‘ mirror ’ reactions D– 3 H and D– 3 He for deuteron energies below 45 keV. The reactions were found to be isotropic and the variations of the total cross-sections over the energy range 20 to 45 keV can be expressed by the simple Gamow theory of penetration of the coulomb barrier. The simple theory assuming the same energy-independent nuclear parameters for the mirror nuclei 3 H and 3 He was unable to account for the observed cross-sections of these two reactions.