Phenolic compounds are promising candidates for use in cosmetic formulations thanks to their characteristics such as antioxidant activity, UV-absorbing and melanogenesis inbibition. But most of them have been shown to be light sensitive with a decrease in their efficacy due to photo-degradation. Therefore, for optimizing the efficacy of functional cosmetics, it is important to recognize the unstable profiles of phenolic compounds in cosmetic products upon ambient sunlight exposure. Four of the most common used phenolic compounds, ellagic acid ( EA ), ferulic acid ( FA ), caffeic acid ( CA ), and chlorogenic acid (CHA), were examined. The phenolic compounds, as a single one and a combination, were formulated into o/w emulsion. And then the chemical stability of each of the phenolic compounds in o/w emulsions with and without the sunlight was investigated. Their different stability was evaluated by a high performance liquid chromatography using an isocratic mobile phase consisting of methanol:acetonitrile:water (10:19:71, v/v/v; pH 3.5). The results demonstrated that the developed mobile phase is very suitable for the simultaneous determination of the four phenolic compounds in emulsions without the interference with the corresponding oxidation products from the phenolic compounds or the excipients from cosmetic emulsions. Moreover, the degradation rate of the phenolic compounds in o/w emulsion is much lower than in solvent, indicating the ability of o/w emulsion to protect the phenolic compounds from rapid degradation. In all cases EA is the most stability due to no degradation product, while CHA has the least stability. In addition, stability of CA was improved markedly when it combined with FA, CHA and EA. This suggests that a suitable combination of phenolic compounds would be required to optimize their efficacy in final products. 天然來源之酚性化合物由於具有好的抗氧化與美白及安全等特性，已成為化妝品產品最佳的原料來源。然而由於這些酚性化合物之化性不安定，因此，當添加在化妝品中時會降低化妝品的安定性與有效性。因此為了獲得健康及優質的功效性化妝品，建立正確的分析方法以及促進酚性物質的安定性是相當重要的。本實驗選擇四種普遍應用在化妝品中的酚性化合物-Ellagic acid(EA)、Ferulic acid(FA)、Caffeic acid(CA) 和Chlorogenic acid(CHA) 分別以單一添加及四種混合添加在基礎乳液中，利用高效液相色層枌析方法進行成份測定及安定性測試。結果顯示，以為移動相可以同時成功地定量乳化製品中所存在的四種酚性物質外，各個酚性物質呈現不同的安定性（在照光與暗室的條件）。其中以EA 最穩定，因為即使在太陽光照射下也不會有任何分解物;但是CHA 則是最不穩定的。另外在FA、CHA和EA 同時存在時可以大大地改善CA 的安定性。因此可知，活性成分被添加在化妝品中時須考慮選擇恰當的組合成分，才能延長活性成分在乳化產品中的安定性與有效性。
