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The alpha-structural transition on slightly non stoichiometric FeS is studied by MossbauerSpectroscopy and Mossbauer Line Tracking methodology [1]. Recent results from different authors lead to uncertainties about FeS transitions that appear in phase diagramhandbooks [2]. According to the most recent reports the superstructure derived from the NiAs-type structure transforms into the MnP orthorhombic one near 400K [3,4]. Both phases coexist within a temperature interval which, depending on sulphur concentration,goes from a few degrees to around 200 K. They are antiferromagnetic with iron atomslocated at unique crystallographic sites. The environment of the probes are dierent enough as to allow hyperne elds from both phases to be distinguished.For a Mossbauer Line Tracking experiment a conventional pulse height selectionbranch, and a Mossbauer driving system in which a novel programmable constant-velocity scaler [5] replaces the usual multiscaler, are used. This set up allows the independent measurement of any channel of a Mossbauer spectrum during an arbitrary period of time.Then, the acquisition of a spectrum region does not necessarily include its central part.Here, this methodology is used to perform a quasi-continuous temperature study ofthe -transition. One or more absorption lines, from both phases, can be followed simultaneously while temperature is varied within the coexistence region. This work is also used as an appropriate test for this novel technique. Therefore, not only the physics of the alpha-transition is treated in this contribution, but a description of the technique and a discussion of the applied line tracking algorithm are also presented.[1] A. Veiga, et. al.,Experimental Design and Methodology for a new Mossbauer scanexperiment: Absorption line tracking, Proceedings ICAME 2007. In press at HyperfineInteract.[2] Massalski, T. B. (1996). Editor. Binary Alloys Phase Diagrams, 2nd ed. plusupdates. Materials Park, Ohio: ASM International.[3] Kyung Seon Baek, et. al. Eects of sulfur vacancies on the crystallographic andspin rotation transition of iron sulde, Phys. Rev. B, 41, (1990) 9024[4] F. Li and H. F. Franze, Phase tranistion in near stoichiometric iron sulde, J.Alloys and Comp., 238 (1996) 73[5] A. Veiga et al., Advances in constant velocity Mossbauer instrumentacion, HyperfineInteract., 167, (2006) 905-909.

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