Acute toxicity of eight phenols towards ten biological test systems is characterized with principal component analysis and partial least-squares regression in terms of underlying modes of action and associated compound properties. The test battery consists of nine different species: three fish, one waterflea, one ciliate, one marine bacterium in two different assays, two fish cell lines and one plant pollen. The compound set contains phenol, five chlorophenols and two nitrophenols, representing polar narcotics and uncouplers of oxidative phosphorylation. Lipophilicity (log K_ow) and acidity (pK_a) as well as respective bilinear decompositions in quantum chemical parameters are used as molecular descriptors. The results reveal substantial differences in specific sensitivity of the test systems for uncoupling activity. Furthermore, pentachlorophenol (as a classical uncoupler) exerts no significant excess toxicity over polar narcosis with any of the ten endpoints, and 4-nitrophenol apparently acts as uncoupler with most of the non-fish test species including procaryotic cells. The discussion includes alternative hypotheses about the underlying mechanism of uncoupling activity.