Minor fluctuations of velocity are frequently observed in gaseous mixtures whose compositions lie near to the limiting values of sustained propagation of detonation. Various possible origins of these fluctuations are discussed. In cylindrical tubes, they commonly take the form of a spinning detonation front. In order to study these effects in detail, previously described methods for making accurate observations near the limits have been extended in various ways. Use of multiple probes set radially in a standard detonation assembly permitted determination of the profiles of spinning ionization fronts to the detonation wave. In some cases, the existence of multiple spin heads detected by these means could be confirmed by soot-trace techniques. For any given mixture, fluctuations in the form of spinning ionization fronts are often remarkably reproducible in a particular tube.