Speciation of the inhibitor lanthanum 4-hydroxy cinnamate in solution as a function of pH and concentration has been evaluated and correlated with corrosion inhibition efficiency for AS1020 steel in an aqueous chloride solution. A combination of nuclear magnetic resonance (NMR) spectroscopy, electrospray mass spectrometry (ESMS), potentiodynamic polarization, scanning electron microscopy (SEM) and profilometric techniques were found to be useful in the characterization of this system. Potentiodynamic polarization experiments established that lanthanum 4-hydroxy cinnamate inhibits well over a range of pH and concentration conditions, but more interestingly the mechanism of inhibition is dependent on the environment. It is also evident from NMR and ESMS that the species in solution are dependent on pH, and this impacts the corrosion inhibition efficiency and mechanism. These data give an insight into the types of species present in a given environment. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.058204jes] All rights reserved.