Abstract: The invention provides a bath for immersion plating a substance with a tin-lead alloy, which includes tin(II) ions, lead(II) ions, organic sulfo groups and thiourea or a mixture of thiourea and a thiourea derivative for a complexing agent. The organic sulfo groups allow insoluble thiourea complexes to be stably dissolved in the solution of the bath; thereby there is no need to retain the bath at a high temperature. Compared with conventional baths for immersion plating, the bath of the invention attains stable plating and a longer life of itself. Addition of the thiourea derivative prevents whiskers from being deposited and allows uniform and sufficiently smooth plating.