Dissolution kinetics of cobalt in liquid 87.5%Sn-7.5%Bi-3%In-1%Zn-1%Sb and 80%Sn-15%Bi-3%In-1%Zn-1%Sb soldering alloys and phase formation at the cobalt-solder interface have been investigated in the temperature range of 250-450 A degrees C. The temperature dependence of the cobalt solubility in soldering alloys was found to obey a relation of the Arrhenius type c (s) = 4.06 x 10(2) exp (-46300/RT) mass% for the former alloy and c (s) = 5.46 x 10(2) exp (-49200/RT) mass% for the latter, where R is in J mol(-1) K-1 and T in K. For tin, the appropriate equation is c (s) = 4.08 x 10(2) exp (-45200/RT) mass%. The dissolution rate constants are rather close for these soldering alloys and vary in the range (1-9) x 10(-5) m s(-1) at disc rotational speeds of 6.45-82.4 rad s(-1). For both alloys, the CoSn3 intermetallic layer is formed at the interface of cobalt and the saturated or undersaturated solder melt at 250 A degrees C and dipping times up to 1800 s, whereas the CoSn2 intermetallic layer occurs at higher temperatures of 300-450 A degrees C. Formation of an additional intermetallic layer (around 1.5 mu m thick) of the CoSn compound was only observed at 450 A degrees C and a dipping time of 1800 s. A simple mathematical equation is proposed to evaluate the intermetallic-layer thickness in the case of undersaturated melts. The tensile strength of the cobalt-to-solder joints is 95-107 MPa, with the relative elongation being 2.0-2.6%.