[en] Guided-ion beam (GIB) measurements of the Ar⁺ + Ar symmetric charge-transfer (SCT) system are presented for ion energies ranging from 0.2 to 300 eV. Two methods are applied to distinguish primary and secondary ions: (i) based on isotopic-labeling, (ii) based on significant laboratory velocity differences. The absolute cross sections measured with these methods are in excellent agreement at energies above 1 eV. The experimental results are compared with semi-classical calculations performed with various published Ar₂⁺ potentials. The calculations including spin-orbit effects lie within 10% of the isotope-selected and attenuation measurements at all investigated ion energies. The present results lie significantly above the simple Rapp and Francis model. Important errors in the latter approach are pointed out and a correct one-electron model is proposed. First measurements of the differential cross section at 0.5 eV collision energy are briefly mentioned. (orig.)