The reaction of 2-methylisoselenochromanium salt with Grignardreagents afforded the reductive ring-opened product by the single electron transfer (SET) mechanism, not by the self-decomposition of σ-selenurane. The same reduction was observed in the reaction of the selenonium salt with magnesium metal. Some other selenonium salts were easily reduced by magnesium metal to give ring-opened products.