Magnetic measurements on Tl x Cr 5 Se 8 (04x41) reveal that stoichiometric TlCr 5 Se 8 is a three-dimensional antiferromagnet with a NeH el temperature T N of about 55 K. In contrast, samples with a reduced Tl content show highly unusual magnetic properties that are without precedent in the literature of magnetically coupled systems: upon lowering the temperature from 300 K the susceptibility reaches a maximum at about 125 K and then steeply drops to a value comparable to that obtained at room temperature. The height of this maximum increases 5rst with decreasing Tl abundance reaching its largest value at a composition Tl 0.2 Cr 5 Se 8 and then decreases again for samples with x(0.2. With decreasing Tl content the e4ective magnetic moment per Cr 3؉ decreases, indicating the successive formation of Cr 4؉ (3d 2 ) centers. For samples Tl x Cr 5 Se 8 with x'0.5 the predominant magnetic exchange is antiferromagnetic and becomes ferromagnetic for x(0.5. The magnetic data suggest a very similar spin structure for all samples except TlCr 5 Se 8 below about 125 K. Low-temperature single-crystal X-ray investigations on Tl x Cr 5 Se 8 (x ؍ 0.5 and 0.32) show a thermal expansion anomaly of the unit cell volume as well as of the b-axis starting at about 125 K, which is accompanied by an enlargement of one of the three Cr+Cr interatomic distances. The anomalous thermal behavior of the lattice parameters re6ects competing antiferromagnetic and ferromagnetic exchange interactions. Detailed analysis of the Cr+Cr distances as a function of temperature gives evidence that the positive charge associated with the formation of Cr(IV) is delocalized between two chromium atoms at higher temperatures and trapped upon cooling on one Cr site. The localization of Cr(IV) centers induces net antiferromagnetic coupling. EPR investigations are in agreement with this picture. They show that in stoichiometric TlCr 5 Se 8 the Cr 3؉ centers are antiferromagnetically coupled. For Tl-poor samples, the temperature dependence of g eff follows the susceptibility curve, increasing upon cooling and passing a maximum at about 130 K.