Two differently hydrated crystal forms of the title compound, viz. bis(acetato-κ²O,O′)(2,9-dimethyl-1,10-phenanthroline-κ²N,N′)mercury(II), [Hg(C₂H₃O₂)₂(C₁₄H₁₂N₂)] or [HgAc₂(dmph)] [dmph is 2,3-dimethyl-1,10-phenantroline (neocuproine) and Ac is acetate], (I), and tris[bis(acetato-κ²O,O′)(2,9-dimethyl-1,10-phenanthroline-κ²N,N′)mercury(II)] hexadecahydrate, [Hg(C₂H₃O₂)₂(C₁₄H₁₂N₂)]₃·16H₂O or [HgAc₂(dmph)]₃·16H₂O, (II), are presented. Both structures are composed of very simple monomeric units, which act as the building blocks of complex packing schemes stabilized by a diversity of π–π and hydrogen-bonding interactions.