Topically applied all-trans retinoic acid (RA) is often associated with skin irritation. A detailed quantification of RA-induced functional changes in stratum corneum is, however, still limited. Using non-invasive bioengineering techniques of measurements of transepidermal water loss (TEWL), stratum corneum hydration and cutaneous blood flow (CBF), we quantified the irritant effects of 0.05% and 0.1% RA in ethanol on normal skin compared with 1% sodium lauryl sulphate (SLS) in water as a model irritant in a 24-h occlusive patch-test assay. Additionally, in order to document data possibly related to the mechanism of action, skin responses to both compounds applied in tandem was also investigated over 18 days. The extent of the irritant response to 0.05 and 0.1% RA, respectively, were similar, implying analogous irritation potency. While RA caused more intense scaling than SLS, other skin responses to RA were significantly weaker than those due to SLS. An increase in TEWL, on day 7, in RA-exposed sites indicates a secondary delayed impairment of the stratum corneum (SC) barrier. In a tandem-design assay, pretreatment with RA appeared to reduce the irritant effects of SLS on SC hydration and CBF. In contrast, pre-exposure to SLS showed a synergestic response in erythema, scaling and TEWL. Our results demonstrate that RA, like SLS, is capable of impairing SC water barrier function, which may be responsible, in part, for the irritation associated with its topical use. However, the distinctive biological responses to these compounds suggest a different mode of action of RA and SLS. In addition, the precise reason for the unique results observed in the tandem-design assays is not clear.