The rate coefficient for radiative and dielectronic recombination of berylliumlike magnesium ions was measured with high resolution at the Heidelberg heavy-ion storage ring TSR . In the electron-ion collision energy range 0–207 eV resonances due to 2 s \to 2 p ( \Delta N = 0 ) and 2 s \to 3 l ( \Delta N = 1 ) core excitations were detected . At low energies below 0.15 eV the recombination rate coefficient is dominated by strong 1 s ^ { 2 } ( 2 s 2 p ^ { 3 } P ) 7 l resonances with the strongest one occuring at an energy of only 21 meV . These resonances decisively influence the Mg ix recombination rate coefficient in a low temperature plasma . The experimentally derived Mg ix dielectronic recombination rate coefficient ( \pm 15 \% systematical uncertainty ) is compared with the recommendation by Mazzotta et al . ( 1998 , A & AS , 133 , 403 ) and the recent calculations by Gu ( 2003 , ApJ , 590 , 1131 ) and by Colgan et al . ( 2003 , A & A , 412 , 597 ) . These results deviate from the experimental rate coefficient by 130 % , 82 % and 25 % , respectively , at the temperature where the fractional abundance of Mg ix is expected to peak in a photoionized plasma . At this temperature a theoretical uncertainty in the 1 s ^ { 2 } ( 2 s 2 p ^ { 3 } P ) 7 l resonance positions of only 100 meV would translate into an uncertainty of the plasma rate coefficient of almost a factor 3 . This finding emphasizes that an accurate theoretical calculation of the Mg ix recombination rate coefficient from first principles is challenging .