| Summary: | When dealing with simple phenols such as caffeic acid (CA) and ferulic acid (FA), found in a variety of plants, it is very important to have control over the most important factors that accelerate their degradation reactions. This is the first report in which the stabilities of these two compounds have been systematically tested by exposure to various different factors. Forced degradation studies were performed on pure standards (<i>trans</i>-CA and <i>trans</i>-FA), dissolved in different solvents and exposed to different oxidative, photolytic and thermal stress conditions. Additionally, a rapid, sensitive, and selective stability-indicating gas chromatographic-mass spectrometric method was developed and validated for determination of <i>trans</i>-CA and <i>trans</i>-FA in the presence of their degradation products. <i>Cis</i>-CA and <i>cis</i>-FA were confirmed as the only degradation products in all the experiments performed. All the compounds were perfectly separated by gas chromatography (GC) and identified using mass spectrometry (MS), a method that additionally elucidated their structures. In general, more protic solvents, higher temperatures, UV radiation and longer storage times led to more significant degradation (isomerization) of both <i>trans</i>-isomers. The most progressive isomerization of both compounds (up to 43%) was observed when the polar solutions were exposed to daylight at room temperature for 1 month. The method was validated for linearity, precision as repeatability, limit of detection (LOD) and limit of quantitation (LOQ). The method was confirmed as linear over tested concentration ranges from 1−100 mg L<sup>−1</sup> (r<sup>2</sup>s were above 0.999). The LOD and LOQ for <i>trans</i>-FA were 0.15 mg L<sup>−1</sup> and 0.50 mg L<sup>−1</sup>, respectively. The LOD and LOQ for <i>trans</i>-CA were 0.23 mg L<sup>−1</sup> and 0.77 mg L<sup>−1</sup>, respectively.
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