Guanine nucleotide exchange factors (GEFs) are crucial regulators of the action of GTPases in signal transduction and cellular regulation. Although their basic mechanism of action has been apparent for almost 20 years, there are still misconceptions concerning their properties, and these are confounded by superficial or incorrect interpretation of experimental results in individual cases. Here, an example is described in which an incorrect mechanism was derived due to an inadequate analysis of kinetic results. In a second example, a case is discussed where certain GTP analogs were erroneously described as being able to function as low molecular weight GEFs. In both cases, lack of distinction between rates, rate constants and apparent rate constants together with disregard of relative signal amplitudes led to the misinterpretations. In a final example, it is shown how the lack of an appropriate kinetic investigation led to the false conclusion that a secreted protein from Legionella pneumophila can act not only as a GEF towards eukaryotic Rab1 but also as a factor that is able to actively dissociate the stable complex between Rab1 and GDI (GDP dissociation inhibitor). This article is protected by copyright. All rights reserved.