Pb-35 wt% Sn alloys were deformed in the semi-solid state. The effects of the initial microstructures, deformation temperatures and deformation rates on the microstructures and the formability of the alloy were studied. A physical phenomenological model is proposed to explain the different behaviours of the stress-strain curves of semi-solid forming of various materials under various conditions. Three different processes, spray deposition (S/D), semi-solid synthesizing (SSS) and conventional casting (CC) were used to produce the Pb-35 wt% Sn alloys with different initial microstructures. Two strain rates, (0.083 and 0.00083 s(-1)) and two deformation temperatures, (190 and 200 degrees C) were used in this study. There is no significant variation in particle size and in sphericity during semi-solid forming of Pb-35 wt% Sn alloys. The deformation stress increases monotonically as strain increases during semi-solid deformation of S/D materials, which consist of smaller solid particles. However, for SSS and CC materials, which consist of larger solid particles, the deformation stress increases in the beginning to a local maximum, then decreases to a minimum before it starts to increase again as strain increases. This phenomenon is more noticeable at lower deformation temperatures and higher strain rates. The deformation stresses required for deformation at higher deformation rates and at lower deformation temperatures are larger than those at lower deformation rates and at higher deformation temperatures, respectively.