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BaCoS 2 is a Mott insulator with a Neél temperature (T N ) close to room temperature. In the ordered antiferromagnetic phase, the magnetic moments are confined in the Co 2 S 2 planes, and arranged in alternatingly directed ferromagnetic chains. We report on resistivity and thermopower measurements and on roomtemperature cell constants of samples of BaCoS z with nominal sulfur content 1.9рzр2.1. We discuss the effect of sulfur content on cell constants and band filling. The main observation is a crossover from activated electronic transport at high temperatures, to high resistivity almost independent of temperature, and accompanied by a vanishingly small thermopower, at low temperatures ͑far below T N ͒. We interpret the transport mechanism in the low-temperature regime as tunneling in the Co 2 S 2 planes, to nearest neighbors along the ferromagnetic chains and to next-nearest neighbors in the perpendicular direction, or alternatively to nearest neighbors along the ferromagnetic chains with conducting inclusions acting as shorts.

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