A method for inhibiting the leaching of mercury from a mercury vapor discharge lamp wherein at least a part of the mercury is present as ionic mercury includes depositing a coating of SnO2 on an interior surface of the lamp envelope. The SnO2 coating is substantially free of a substance which would cause electrical conductivity of the coating. Further included within the lamp is a quantity of oxidizable iron in an amount equal to at least 1 gram per kilogram of lamp weight.