The polymorphism of bulk powder samples of potassium ferrocyanide trihydrate (K4Fe(CN)(6)center dot 3H(2)O, KFCT) has been studied using H-1, C-13, and N-15 NMR spectroscopy in combination with X-ray diffraction. At room temperature, KFCT typically crystallizes in a monoclinic C2/c form, which converts irreversibly to a monoclinic Cc form upon cooling below -25 degrees C. The structure of both of these forms has been determined using single-crystal X-ray diffraction. A less common metastable tetragonal l4(1)/a form is also known to exist at room-temperature. This tetragonal form also converts to the monoclinic Cc form upon cooling, although this phase transition is irreversible and occurs at -60 degrees C. Initial room-temperature N-15 MAS NMR spectra and powder X-ray diffraction patterns of ground powder samples of KFCT prepared using a variety of crystallization methods suggested that only the C2/c form was obtained from a bulk crystallization. The C-13 MAS NMR spectra consisted of six peaks with equal integrated areas, a result that is inconsistent with the N-15 NMR spectra and known crystal structures. When the samples were not ground, the relative areas of the C-13 NMR peaks were altered, indicating that the bulk samples in fact consisted of the two known forms of KFCT. Using the known temperature dependence of these two polymorphs, the C-13 peaks corresponding to each of the C2/c and l4(1)/a forms were assigned. The C-13 NMR spectra and powder X-ray diffraction results demonstrate that upon grinding, a near 50-50 mixture of the two forms is always produced, rather than a new form entirely. The insensitivity of the N-15 NMR spectra to the polymorphism of KFCT is surprising, and likely arises from a fortuitous overlap of the N-15 NMR peaks of the two forms.