A much improved synthetic route to malonic acid half thioesters (MAHTs) and oxyesters (MAHOs), the easy incorporation of deuterium labeling expecially in MAHTs, and an unexpectedly large difference in enolization chemistry between MAHTs and MAHOs are reported. Density functional theory calculations explore the origins of this difference and identify an additional MAHT molecular orbital which increases delocalization between sulfur and the enol in both cisoid and transoid forms.