In the present investigation, the corrosion behaviour of dissimilar A319 and A356 cast Al alloys plates joined by friction stir welding (FSW) was evaluated. The effects of tool rotational and welding speeds as well as post-weld heat treatment (PWHT) on corrosion behaviour were investigated. Plates of A319 and A356 were friction stir welded (FSWed) using three different tool rotational speeds (typically, 1120, 1400 and 1800 rpm) and two welding speeds (typically, 80 and 112 mm/min). The PWHT was carried out using a solution heat treatment temperature of 540 oC for 12 hours followed by ageing at 155 °C for 6 hours. Corrosion behaviour of welds was investigated by immersion in sodium chloride (NaCl) and hydrogen peroxide (H2O2) solution for 6 hours. The results showed that both as-welded (AW) and PWHTed welds showed better corrosion resistance than both A319 and A356 base alloys. The corrosion resistance of the welded zones was found to be reduced by increasing the tool rotational speed and/or reduction of the welding speed.