Sphalerite, the only major source of zinc, is present in significant quantities in many metamorphosed ores which occur along the Appalachian-Caledonian orogen. Ore samples from more than 20 deposits representing a variety of geologic environments in the Norwegian Caledonides have been studied by electron microprobe analysis to investigate the compositional means and ranges of the sphalerites. Most specimens consisted of zinc concentrates and thus allowed a representative sampling of sphalerites from throughout the deposits.The mean iron contents of the sphalerites from individual deposits range from less than one weight percent to more than 11 weight percent and correlate with the nature of the coexisting iron sulfides which have served to buffer the a (sub S 2 ) , and hence the a FeS in the sphalerite. Thus deposits containing pyrite but not pyrrhotite contain sphalerites with very low iron contents and narrow compositional ranges. The deposits with irregularly dispersed pyrite and pyrrhotite have sphalerites with higher and more variable iron contents. Sphalerites from those deposits which contain uniformly distributed pyrite and pyrrhotite have high iron contents but exhibit narrow compositional limits. The cadmium contents in sphalerites range between 0.1 and 0.4 weight percent but reveal no trends with the iron contents of the sphalerites. In contrast, the manganese contents of the sphalerites, which range from 0.0 to 0.5 weight percent, increase with increasing iron content.