Ab initio molecular dynamics simulations and in situ Raman spectroscopy are used to study the speciation in two molal aquecus MgSO4 solutions at high pressures, P, and temperatures, T. While at ambient conditions the fluid is dominated by dissociated SO42-(aq) ions and solvent-separated ion pairs, ion association strongly increases with increasing temperature and pressure along a 1.33 g/cm(3) isochore. At T = 450 degrees C and P = 1.4 GPa, the v(1)(SO42-) Raman band is well described by three Gaussian + Lorentzian components of about equal intensity with peaks at about 980, 995, and 1005 cm(-1). Analysis of the simulations, however, indicates the coexistence of more than three species, including dissociated SO42-(aq) ions, and contact and triple ion pairs as well as larger complexes. In addition, the sulfate groups may be bonded to Mg as monodentate or bidentate ligands. The frequencies of the associated species seem to depend mainly on the type and number of Mg-SO4 bonds. We interpret the two rather broad high-frequency Raman components as a single "Mg-SO4 contact" component with variable frequency distribution. As a consequence, the V-1(SO42-) Raman band provides only information on the molecular environment of the sulfate group; i.e., individual species cannot be resolved. At fluid densities less than about 1.2 g/cm(3) and temperatures above 400 degrees C, the formation of HSO4-(aq)-containing species is observed in both simulations and experiments, which may be accompanied by a change in pH and electrical conductivity.