Aldehydes are produced endogenously in the human liver as a result of lipid peroxidation, suggesting the liver must have mechanisms for dealing with these reactive molecules. In addition, certain herbal products and natural oils contain significant amounts of aldehydes. It is therefore important to recognize potential interactions between these compounds and metabolic systems in the body. We have looked at the metabolism of a series of long chain saturated and unsaturated aldehydes in human liver microsomes using NADPH as an electron donor. Two main products are formed; the alcohol (reduction) and carboxylic acid (oxidation). Chain length of the aldehydes and saturation in the chain do not appear to be major determinants of activity with regard to alcohol formation over the carbon chain range of 9-12. Inhibition of human cytochrome P4502E1 by these aldehydes was observed. Here there did appear to be a correlation between potency of inhibition and chain length. Finally, a series of popular essential oils, many of which have high aldehyde content, were examined with regard to their ability to inhibit the P4502E1. At a concentration of 0.005 microgram/mL, inhibition ranged from >90% to 0% indicating a high degree of specificity in the inhibitory effect.