The role of tin (Sri) in assisting the nucleation of the plate-shaped precipitate phase theta' in an Al-4Cu-0.05Sn (wt%) alloy has been investigated by high resolution transmission electron microscopy (HRTEM) and three-dimensional atom probe field ion microscopy (3D-APFIM). HRTEM revealed that the strong contrast on the rim of many theta' precipitates in underaged alloys may be attributed to formation of a Sri particle, possibly the beta-Sn phase, with an orientation relationship with respect to the aluminium matrix that is different to that commonly accepted. 3D-APFIM revealed local zones containing an enrichment of Sri atoms, but a majority of aluminium atoms, suggesting that it is small clusters of Sri atoms, and not the beta-Sn phase, that are involved in lowering the nucleation barrier for the theta' phase. This interpretation is supported by the HRTEM observation that, even before nucleation of most of the theta' phase, very few Sri particles are found independent of theta' precipitates.