The preparation, crystal structures, and electric and magnetic properties of (BETS)(2)MX(4) molecular conductors (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Mn2+, Fe3+, Co2+, Ni2+, Cu2+; X = Cl, Br, CN) are reported. Resistivity measurements down to 2 K reveal the coexistence of the BETS pi conduction electrons and the localized magnetic moments of the anions in the K-(BETS)(2)FeCl4 and kappa-(BETS)(4)(CoCl4)(C2H3Cl3) salts. Another FeCl4 phase, lambda-(BETS)(2)FeCl4, undergoes a sharp metal-insulator (MI)) transition around 8 K. At the same temperature, a magnetic transition of the FeCl4-anions takes place cooperatively. A superconducting transition is observed at 4.6 K in lambda-(BETS)(2)(FeCl4)(0.5)(GaCl4)(0.5), where half of the anion sites are occupied by magnetic ions. The crystals prepared from 1,1,2-trichloroethane solutions with the NiCl42- and MnCl42- anions exhibit the behavior of a semimetal down to approximate to 100 K. The (BETS)(4)(Cu2Cl6) salt remains metallic down to 4.2 K. ESR studies show that the Fe3+ ions in kappa- and lambda-(BETS)(2)FeCl4 are in a high-spin state. The temperature dependencies of the spin susceptibilities of kappa- and lambda-(BETS)(2)FeCl4 indicate antiferromagnetic interactions between the Fe3+ ions. The crystal structure analyses of kappa- and lambda-(BETS)(2)FeCl4 have been carried out at 298 and 10 K. Closer BETS ... FeCl4 contacts in lambda-(BETS)(2)FeCl4 are observed, which is consistent with the larger Weiss temperature of this compound.