The sequential bond energies of Mg2+(H2O)x complexes, in which x=2–10, are measured by threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer. From an electrospray ionization source that produces an initial distribution of Mg2+(H2O)x complexes in which x=7–10, complexes down to x=3 are formed by using an in-source fragmentation technique. Complexes smaller than Mg2+(H2O)3 cannot be formed in this source because charge separation into MgOH+(H2O) and H3O+ is a lower-energy pathway than simple water loss from Mg2+(H2O)3. The kinetic energy dependent cross sections for dissociation of Mg2+(H2O)x complexes, in which x=3–10, are examined over a wide energy range to monitor all dissociation products and are modeled to obtain 0 and 298 K binding energies. Analysis of both primary and secondary water molecule losses from each sized complex provides thermochemistry for the sequential hydration energies of Mg2+ for x=2–10 and the first experimental values for x=2–4. Additionally, the thermodynamic onsets leading to the charge-separation products from Mg2+(H2O)3 and Mg2+(H2O)4 are determined for the first time. Our experimental results for x=3–7 agree well with quantum chemical calculations performed here and previously calculated binding enthalpies, as well as previous measurements for x=6. The present values for x=7–10 are slightly lower than previous experimental results and theory, but within experimental uncertainties.