  At present three pulsars are known which clearly show a strongly increasing
degree of circular polarization with frequency. As this is accompanied by a
smooth decrease of linear polarization, we investigate, if this observation can
be explained through a propagation mode transition from linear to circular
polarization. Using a previously published model we find that the rate of
change of the polarization types with frequency is well consistent with the
theory. The small number of only three objects does of course not allow to draw
a general conclusion for the whole sample of pulsars. However we show that the
very unusual behavior of these three objects can be well explained with this
model and we discuss, why only so few such objects are presently known.
