The relation between crystal structure and bulk magnetic properties is investigated in the molecular charge transfer salts (BEDT-TF)(2)MCl(4) (M = Ga, Fe). (BEDT-TTF)(2)GaCl4 crystallizes in the triclinic system. Its crystal structure consists of pairs of BEDT-TTF molecules arranged in layers with intermolecular S ... S interactions. Band structure calculations predict semimetallic behavior contrary to the semiconductivity observed even under a pressure of 6 kbar (sigma(300 K, 1 bar) = 10(-1) S cm(-1) and E(A) = 0.2eV). The static (Faraday and SQUID magnetometry) and spin (EPR) susceptibilities indicate low-dimensional Heisenberg antiferromagnetic behavior with the susceptibility tending to zero as the temperature approaches zero. The data are analyzed using several low-dimensional magnetic models and are best fitted to a model consisting of two different spin dimers (Delta(1) = 108 K and Delta(2) = 212 K). The static magnetic susceptibility of (BEDT-TTF)(2)FeCl4 is modeled by a sum of Curie-Weiss (S = 5/2 for Fe(d(5)) and Theta = -4 K), chi(tip), and single dimer (Delta = 45 K) parameters. The BEDT-TTF layers in these compounds thus behave as Mott-Hubbard-localized systems, and the interaction between the magnetic moment on the Fe with those on the organic layer is negligible.