relaxation times of concentration fluctuations in semi-dilute solutions of polydimethylsiloxane, have been measured at several values of the reciprocal wave vector q, with the neutron spin echo spectrometer of the laboratoire leon brillouin. the experiment has been carried out successively on a solute of identical chains, and on a solute divided in equal parts between labelled and nonlabelled chains at zero average contrast. the authors report observations of the dispersion relation associated with the brownian motion of the polymer chains, in the vicinity of the inverse mesh size 1/ xi . for values of q which are greater than 1/ xi , the two experiments give identical results but when 1 decreases below 1/ xi , the dispersion curves associated with each experiment are different. a bifurcation occurs at 1/ xi and two distinct transport processes become observable, which are related to cooperative diffusion and to inter-diffusion respectively. the observed coefficients are compared with predictions of the effective medium and the dilute solution theories