A tin or tin-lead alloy electroless plating bath comprising (A) a stannous salt or a mixture of a stannous salt and a lead salt, (B) an acid, (C) thiourea or a thiourea derivative, and (D) a reducing agent is improved by adding (E) a nonionic surfactant and optionally, (F) a cationic surfactant, a nitrogenous heterocyclic compound or a derivative thereof, or alternatively by adding (G) an ammonium or quaternary ammonium ion. From the bath, uniform films of fine grains will chemically deposit on fine pitch printed wiring boards intended for SMT.