The grain boundaries of a fibrous Al-Mg-Si-Cu alloy have been investigated with Transmission Electron Microscopy. The compositions have been mapped by Energy Dispersive X-ray Spectroscopy. The alloy has been aged for 12 hours at 155°C after solution heat treatment and is in a slightly underaged condition. The precipitates nucleated on the high angle grain boundaries are coarse, while the precipitates on the low angle grain boundaries are smaller and more numerous. The precipitates on both types of grain boundaries has been identified as Q'-type. Copper is segregated to both the low and high angle grain boundaries. The effect of this segregation will be discussed with regards to the corrosion properties of the alloy.