Abstract The photostability of guaiazulene (1,4‐dimethyl‐7‐isopropylazulene; GA), a natural azulenic compound used in cosmetic and health‐care products, as well as in pharmaceutical preparations, was investigated in solution (methanol, ethanol, acetonitrile), by different techniques: gas chromatography/mass spectrometry (GC/MS) and high‐performance liquid chromatography combined with atmospheric pressure chemical ionization mass spectrometry and UV detection (LC/APCI‐MS and HPLC/UV). A solar simulator (xenon‐arc lamp) was used as UV‐A radiation source. The study involved: monitoring compound decomposition, identifying products of photodegradation (PPs), assessing the role of oxygen and evaluating the kinetics of the process. Minor PPs are volatile compounds and were characterized by GC/MS, while oligomeric polyoxygenated compounds, tentatively characterized on the basis of MS and MS/MS spectra, were found to be the main photoproducts. The photodegradation was found to be enhanced by the presence of oxygen; nevertheless, determination of the singlet oxygen quantum yield for GA gave a lower value than that for the reference standard Rose Bengal. The obtained results and the developed stability‐indicating methods (GC/MS and LC/MS) are of interest for stability studies and/or quality control purposes of GA as raw material or cosmetic products. Copyright © 2008 John Wiley & Sons, Ltd.