The radiative lifetime and mass isotope shift of the 1s²2s ²2p ²P_3/2 - ²P_1/2 M1 transition in Ar¹³⁺ ions have been determined with high accuracies using the Heidelberg electron beam ion trap. This fundamentally relativistic transition provides unique possibilities for performing precise studies of correlation and quantum electrodynamic e.ects in many-electron systems. The lifetime corresponding to the transition has been measured with an accuracy of the order of one per thousand. Theoretical calculations predict a lifetime that is in signi.cant disagreement with this high-precision experimental value. Our mass shift calculations, based on a fully relativistic formulation of the nuclear recoil operator, are in excellent agreement with the experimental results and con.rm the absolute necessity to include relativistic recoil corrections when evaluating mass shift contributions even in medium-Z ions.