The microstructures of low carbon steels with Sn additions were investigated using scanning electron microscopy, electron probe microanalysis, transmission electron microscopy and energy dispersive X-ray spectroscopy. Four steels based on Fe-0.9Nb-0.3Sn-0.05C (wt%) with different levels of Al and Si additions were prepared by arc melting under an argon atmosphere. The effects of heat treatment and the level of alloying elements Al and Si on the precipitation of Sn-rich phases were studied. After ageing at 1150 degrees C and 850 degrees C, NbC precipitates were found in all samples, as well as AlN in the higher Al content steels. The concentration of Al in steel was also found to affect the formation of Sn-rich compounds after heat treatment at 850 degrees C for 96 hours. In the lower Al or Al-free steels, a eta-Fe2Nb3 phase, which dissolves a significant amount of Si, was observed. In the higher Al steels, a Fe2Nb-based Laves phase, which dissolves both Si and Sn was detected. A mechanism based on both size factors and thermodynamic considerations is described, which accounts for the experimental observations.