Abstract: A tin-lead alloy plating bath based on a principal plating bath comprising an alkanesulfonic or alkanolsulfonic acid and both bivalent tin and lead salts thereof is characterized by the addition of a guanamine compound having the general formula ##STR1## wherein R.sub.1 and R.sub.2, which may be the same or different, represent each a hydrogen atom, C.sub.1-18 straight- or branched-chain alkyl radical, C.sub.1-18 straight- or branched-chain alkoxy-lower alkyl radical, or a C.sub.3-7 cycloalkyl radical, or R.sub.1 and R.sub.2 may combine to form a carbon cycle or hetero cycle, and A represents a lower alkylene radical.