Doping of spin-ladder systems by isostructural paramagnetic complexes was attempted. Despite the close isostructural nature of the pure (DT-TTF)(2)[M(mnt)(2)] (M = Au, Ni, Pt) end-members, which present a ladder structure, doping of the spin-ladder (DT-TTF)(2)[Au(mnt)(2)] with either 5% or 25% [M(mnt)(2)](-) (M = Ni, Pt) generates two (metrically) new phases. Their markedly different crystal structures have been determined using laboratory X-ray powder diffraction data. (DT-TTF)(2)[Au0.75Ni0.25(mnt)(2)] consists of a mixed-valence compound (of triclinic symmetry), which was only detected, pure or in a mixture of phases, when [Ni(mnt)(2)](-) was used as a dopant. Differently, the stoichiometric 1:1 [DT-TTF][Au0.75Pt0.25(mnt)(2)] monoclinic phase was found when [Pt(mnt)(2)](-) (in 5% and 25%) was employed as the doping agent. Remarkably, only in the 5% Pt doping experiment, the major component of the mixture was the ladder structure compound (DT-TTF)2[Au(mnt)(2)] doped with minor amounts of Pt. This 5% Pt-doped specimen shows an EPR signal (g = 2.0115, Delta H-pp = 114 G at 300 K) wider than the pure compound (DT-TTF)(2)[Au(mnt)(2)], denoting exchange between the donor spins and Pt(mnt)(2-) centers. The electrical transport properties of the 5% Pt-doped composition at high temperatures are comparable to those of (DT-TTF)(2)[Au(mnt)(2)] with room-temperature conductivity sigma(300K) = 13 S/cm and thermopower S-300K = 46 mu V/K, with a sharp transition at 223 K similar to that previously observed in the Cu analogue at 235 K.