Trace elements of blocky K-feldspar and muscovite have been tested as a prospecting method for Sn- and Li-mineralized pegmatites in Sweden. There is a strong correlation between elevated amounts of Sn in muscovite, > 500–600 ppm, and the presence of cassiterite. However, a combination of high Sn and low Mg with moderate to low Fe tot, and especially negligible Fe 3+, in muscovite seems to be a more reliable indicator for the presence of cassiterite in a granitic pegmatite. In barren pegmatites, muscovite with elevated Fe 3+ (5–10% of Fe tot) can apparently be Sn-rich even in the absence of cassiterite. The method merely indicates the presence but not the amount of cassiterite. Elevated contents of Li (> 80 ppm) in K-feldspar are a more reliable indicator for spodumene-bearing pegmatites than Li in muscovite. In less fractionated pegmatites muscovite is probably the only Li-bearing mineral. Y is considerably higher in muscovites of the gadolinite-type pegmatites than in those of lithium pegmatites, and can be used as an indicator for gadolinite-type pegmatite fields.