Electro-oxidation of a solution of [NBu(4)][Ni(dmit)(2)] (dmit = C3S52- = 2-thioxo-1,3-dithiole-4,5-dithiolate) in the presence of a large excess of guanidinium tetrafluoroborate (GuaBF(4)) or 1,1-dimethylguanidinium tetrafluoroborate (Me(2)GuaBF(4)) yields black crystals of [Gua][Ni(dmit)(2)](2) (1) and [Me(2)Gua][Ni(dmit)(2)](2) (2), respectively. The molecular structures of 1 and 2 have been determined by X-ray crystallograghy. Crystal data for 1 : triclinic, space group P $($) over bar$$ 1 (No. 2) with a = 6.472(3) Angstrom, b = 7.420(4) Angstrom, c = 33.468(5) Angstrom, alpha = 87.97(3)degrees, beta = 85.80(3)degrees, gamma = 70.96(4)degrees, V = 1515.1(12) Angstrom(3), and Z = 2. Crystal data for 2 : triclinic, space group P $($) over bar$$ 1 (No. 2) with a 8.9864(12) Angstrom, b = 10.0138(9) Angstrom, c = 18.238(2) Angstrom, alpha = 81.963(9)degrees, beta = 89.071(10)degrees, gamma = 72.447(9)degrees, V 1548.8(3) Angstrom(3), and Z = 2. In compound 1 two crystallographically independent Ni(dmit)(2) units, Ni(1)(dmit)(2) and Ni(2)(dmit)(2), are lying in a head-to-tail orientation forming two different sheets of units in the ab plane. In the Ni(1)(dmit)(2) sheet the units are slightly dimerized and a two-dimensional network of short S...S contacts is formed. The Ni(2)(dmit)(2) sheet shows no dimerization, and a one-dimensional network can be seen. The sheets are connected along the c-axis via short S...S contacts of the thione sulfur atoms and via N-H...S hydrogen bonds with the cations which are lying in between. In compound 2 two crystallographically independent Ni(dmit)(2) units are lying in a face-to-face orientation thereby forming one type of anionic sheet in the ob plane. A two-dimensional network of short S...S contacts is formed by slightly dimerized Ni(dmit)(2) units in the sheet. Intersheet contacts are present through short S...S contacts and N-H...S hydrogen bonds as in compound 1. For both compounds intermolecular orbital overlap integral calculations point toward a weak two-dimensional conduction pathway in the ab plane. Conductivity measurements indicate a room temperature value of 32 S.cm(-1) for 1 and 0.15 S.cm(-1) for 2. Both compounds behave as semiconductors in the temperature range of 300-100 K with E(a) = 0.12 and 0.13 eV for 1 and 2, respectively.