Metaphosphate is known to be highly reactive to water, whereas thiometaphosphate is relatively stable in aqueous solution. The difference in their reactivity has important mechanistic implications in interpreting the "thio effect" in phosphoryl transfer reactions. In this work, density functional theory is used to investigate the reactivity of both metaphosphate and its thio-substitute in their complexes with one, two, and three waters, and in aqueous solution. Barrier heights for converting metaphosphate to orthophosphate have been determined by geometry optimization. The results confirm that metaphosphate is consistently more reactive than thiometaphosphate and the activation free energy for both species decreases with the number of water molecules. The relative stability of thiometaphosphate is attributed to its less positively charged phosphorus atom.