Mg-3wt.%RE alloys show substantially enhanced < c + a > activity as compared to pure Mg or other Mg-Al, Mg-Zn alloys of similar grain sizes. Activation of < c + a > slip has been postulated to be associated with the reduction of the basal Li stacking fault (SF) energy upon alloying with RE elements. However, the underlying mechanism and any special role, of the RE solutes, as compared to Al and Zn, remain unclear. Here, precise first-principles methods and calculations are used to assess all relevant SF energies as a function of V, Al and Zn concentrations in the dilute limit relevant to current experiments. Results show that solute effects vary among different solutes and among different SFs of the same solute. However, the effects of Y on basal plane SF energies can also be achieved by Al at similar/twice the concentrations and the effect of Y on the pyramidal (Pyr.) II plane can be achieved by Zn at similar concentrations. Enhanced < c + a > activity in Mg-3wt.%RE alloys thus does not appear to be directly related to these SF energies, in contrast to current widely-accepted postulates. The average reductions in Pyr. II and I SF energies by Y also have little influence on the strong thermodynamic driving force for the transition of easy-glide Pyr. II and I dislocations into sessile basal-plane-dissociated structures. Y solutes only stand out in affecting the relative pyramidal SF energies differently than Al and Zn, which is not considered in any mechanisms proposed to date. These results highlight the importance of precise first-principles calculations in study of solid solution alloying at appropriate solute concentrations, which allows for the quantitative assessment of potential dislocation phenomena related to ductility. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.