We have synthesized two forms of erbium tetracyanoplatinates, Er-2[Pt(CN)(4)](3). 21H(2)O (red form) and Er-2[Pt(CN)(4)](2). SO4. 11.5H(2)O (yellow form), and determined their crystal structures by X-ray diffraction. While the red form crystallizes in the orthorhombic space group Pbcn, with a = 15.4848(3) Angstrom, b = 13.8186(2) Angstrom, c = 19.07820(10) Angstrom, alpha = beta = gamma = 90 degrees, and Z = 4, the yellow form precipitates in the tetragonal space group 14cm, with a = b = 14.321(2) Angstrom, c = 13.338(3) Angstrom, alpha = beta = gamma = 90 degrees, and Z = 4. Both forms show [Pt(CN)(4)](2-) chains but differ markedly in color and morphology. This is due to the incorporation of sulfate ions in the latter modification, leading to an increased Pt-Pt distance. The observed optical absorption and emission behavior of the title compounds is correlated with the Pt-Pt distances.