Inelastic collision processes between neutral Mg atoms and Rb+ ions, both in their ground states, have been studied by means of a crossed molecular beam technique measuring the decay fluorescence of the excited species formed. Emissions corresponding to Mg(3 P-1(1)), Mg(3 D-3(3,2,1)), and Mg(4 S-3(1)), formed by direct target excitation, Rb(5 P-2(3/2,1/2)), Rb(6 P-2(3/2,1/2)) produced by electron capture and also the phosphorescent emission due to decay of Mg(3 P-3(1)), have been detected and the corresponding absolute cross-section values measured both as total values and resolved into their J states. No polarization measurements could be made. Ab initio calculations using pseudopotentials have been performed and from these a manifold of adiabatic energy curves correlating with the different entry and exit channels have been obtained, allowing to propose a qualitative interpretation of the results, such as the shape of the cross section vs energy for different transitions and the oscillating nature of the branching ratios due to interference effects. (C) 2004 American Institute of Physics.