The susceptibility of the aluminium alloy, AA2024-T3, to localised corrosion as well as the toxicity of chromate inhibitors are both well known facts. In this paper the effect on the corrosion of AA2024-T3 of the thiol-containing mercaptoacetate, when coupled with cerium or praseodymium is examined. Potentiodynamic polarisation and SEM/EDXS were used to analyse the inhibiting behaviour of the compounds cerium mercaptoacetate and praseodymium mercaptoacetate. Both inhibitors were predominantly cathodic inhibitors with increasing anodic inhibition after 24 h of immersion. A mechanism of film formation and corrosion protection was also proposed.