Abstract Micrometer-size condensate particles consisting of zincite (ZnO) with possibly minor zinc silicate occur in the phreatic ashes of Mt. St. Helens that were deposited in early April 1980. Zincite probably results from the reaction between steam, air and magmatic zinc halide. Thermodynamic calculations indicate that oxidation of zinc halide proceeds under the prevailing conditions in the eruption cloud. The zinc enrichment in the phreatic ashes adds evidence to the hypothesis that magmatic vapors were involved in the phreatic eruptions. It appears as if zinc is released from the magma already in the earliest stages of volcanism; monitoring of zinc in fumaroles and hot springs could possibly be helpful in the prediction of volcanic activity.