With the development of multi-collector inductively coupled plasma mass spectrometry, the stable isotopes of Mg have become available as a tool to constrain Mg cycling within the weathering environment. This advance is important, as Mg isotopes will allow the study of Mg transport within ecosystems and soils directly and may also lead to insight into Mg weathering and CO2 cycling on geological timescales. Studies investigating the stable isotopes of Mg in soils, waters, and plants show that significant Mg isotopic fractionations are associated with weathering, secondary mineral formation, and biotic uptake. However, studies identifying Mg isotopic fractionation have yet to divide soils into detailed fractions to locate those isotopic fractionations in specific soil phases, and studies have yet to utilize the soil chronosequence concept to determine if the Mg isotopic composition of soil changes through time. We address these issues by conducting bulk soil Mg isotopic analyses along with a detailed sequential extraction on an arid Hawaiian soil chronosequence to determine the distribution of Mg and Mg isotopic composition within soil phases. We find that the overall Mg isotopic compositions of our soil systems are determined by the secondary phases that dominate the functioning of each particular soil system. Our younger chronosequence site is dominated by pedogenic carbonate, leading to the net export of isotopically heavy Mg from this soil and the retention of isotopically light Mg, while our older chronosequence site is dominated by non-carbonate phases, leading to the export of Mg near the Mg isotopic composition of basalt. This evidence illustrates that as secondary mineral phase assemblages evolve over time in soils, the isotopic composition of the Mg exported from soil systems can also change. Further, by looking at the detailed sequential extraction fractions, we see that Mg in the adsorbed cations and carbonates is isotopically lighter than the Mg inputs to our soils, and the Mg associated with soil organic matter, short-range-order phases, sesquioxides, and residuum is near the Mg isotopic composition of basalt or isotopically heavier.