It has generally been assumed that increasing atmospheric CO2 concentrations will increase plant carbon-based secondary or structural compounds concentrations. These changes may have far-reaching consequences for herbivory and plant litter decomposition. Recent experimental results provide evidence of increases in concentrations of soluble phenolics and condensed tannins but not in lignin, structural polysaccharides or terpenes. They also show significant effects of these plant chemical changes on herbivores and little or any effects on decomposition. However, there is no consistent evidence of any of these effects at the complex long-term ecosystem level.