Photoisomerization of trans-stilbene and its derivatives is known to proceed via an activation step from the S-1 (and in some cases also T-1) state. Accordingly, the activation energy of this process can in principle be determined either by measuring the quantum yields of isomerization directly or by analyzing the temperature behavior of the fluorescence quantum yield (Arrhenius plot) instead. Since both techniques are expected to produce the same result, many authors prefer the latter because of easier feasibility. In this letter, we show that at least for some donor substituted trans-stilbenes, this is not justified: parallel measurements of fluorescence and isomerization quantum yields of trans-4,4'-diaminostilbene and trans-4,4'-di(phenyl-ureanyl)-stilbene as a function of temperature exhibited a decrease of the quantum yields of fluorescence and isomerization with increasing temperature. We deduce the presence of a second activated process, responsible for reducing phi (f) and phi (t -->c) and for governing the anomalous temperature dependence of phi (t -->c).