We have investigated the effeet of the preeipitation proeess appearing after the annealing of lead--tin alloys on the critical temperature and on the magnetic properties. We examined the occurrence of the flux-jump state, its dependence upon the precipitation and the temperature, giving a theoretical model for this process. The Nernst--Ettingshausen effect has been investigated for deepl T doped samples. We deduced the character of the electron--phonon and pinning--vortex interactions.The lead--tin metallic system almost in the entire range of concentration is a superconductor of type II.As is shown in the phase diagram (Fig. 2) the Pb--Sn system can forma solid solution from 0 to 29 at % of Sn. Decreasing the temperature, this one phase alloy is separated by the selection of Sn precipitations to a 2-phase system.This structural change appreciably influences the superconducting parameters. We investigated this process measuring the magnetisation of samplr containing 2.5; 5; 10 and 15 at% of Sn. Our cylindrical samples hada length of 60 mm anda diameter of 4 mm. The samples were prepared by the following temperation processes: 200 ~ 10rdn quenched in (iquid nitrogen A B He temperature room temperature lh Fig. 1. Temperation process *State University of Alexandria, Egypt 4 Avta Physiea .4cademiae Svientiarum Hungaricae 50, 1981