Attenuated total reflectance infrared (ATR-IR) and Raman spectroscopies are used to identify the complexed species of mercury(II) with acetate and some aliphatic derivatives in aqueous solutions, as a function of pH and metal-to-acetate ratio. The spectra of the free carboxylate ions, in their sodium salt solutions, and of the protonated molecules are shown for comparison and assigned below 1800 cm−1. Whatever the stoichiometries assumed [HgCH3COO+ up to Hg(CH3COO)42-], one structure is observed, principally via the stretching modes ν(CO2) and ν(CC), although two strong bands have a νs(CO2) character owing to an accidental coupling of this mode with δs(CH3) in all the acetate complexes. This coupling is demonstrated with the isotopic ligand CD3COO, which displays only one νs(CO2) band on account of the separation of this mode from δs(CD3). The splitting between νa(CO2) and νs(CO2) increases up to about 260 cm−1 for all the carboxylates under study, the value pointing to a hydrated monodentate form.