The colorants alizarin and Alizarin Crimson (a calcium-aluminum lake pigment), and their simple structural homologues anthraquinone were deposited on silica gel, cellulose, and Teflon substrates, and exposed in the dark to ozone in purified air ( ~ 0.403 for 95 days). Exposed and control samples were analyzed by mass spectrometry. Alizarin Crimson reacted with ozone on all substrates, yielding phthalic acid (major), benzoic acid (minor), and other minor and unidentified products. Anthraquinone did not react with ozone irrespective of conditions. Alizarin did not react on Teflon or cellulose but reacted on silica gel to yield phthalic acid (major) and other products. A chemical mechanism responsible for the fading of these alizarin-based colorants by ozone is suggested that is consistent with the products distribution, the observed reactivity sequence, and the observed substrate-specific effects. The possible application of this work to predicting the ozone fastness of other alizarin-related pigments is discussed briefly. -- AATA