  The composition and site occupancy of Mg within ordered {\delta} precipitates
in a model Al Mg Li alloy have been characterized by atom probe microscopy and
first-principles simulations. The concentration in the precipitates is found to
be almost the same as that of the matrix, however, we show evidence that Mg
partitions to the sites normally occupied by Li in the L12 structure. Density
functional calculations demonstrate that this partition is energetically
favorable, in agreement with experimental results.
