A barrier to radial O2 loss (ROL) is an adaptive trait of many wetland plants, yet the signal(s) for barrier induction remain uncertain. We assessed the effects of monocarboxylic acids produced in waterlogged soils (acetic, propionic, N-butyric and caproic acids) on barrier formation in adventitious roots of the waterlogging tolerant Hordeum marinum Huds. These acids were applied in nutrient solution either individually (at 0.4mM) or as a mixture ('cocktail') at various total concentrations (0.1, 0.4 and 2mM) at pH 6. The barrier to ROL was formed in basal zones of roots exposed to the cocktail at 0.4mM, but not at 0.1mM. Individually, only N-butyric and caproic acids invoked a 'tight' barrier in subapical positions of the roots. These organic acids accelerated deposition of suberin in the hypodermis/exodermis, but did not affect overall root porosity (% gas space). The organic acids also reduced root extension rate and tissue K+; effects were more pronounced at higher concentrations of the cocktail and as the molecular weight of the organic acid increased. Moreover, the cocktail at 2mM and caproic acid at 0.4mM alone induced development of intercellular occlusions, suggesting phytotoxin injury. In summary, even relatively low concentrations of organic acids can promote barrier formation in roots, and the potential toxicity of these compounds was demonstrated by declines in root growth and tissue K+ in the wetland species H. marinum. © 2014 CSIRO.