Solutions of polyisobutylene (PIB) in ethyl caprylate were investigated using mechanicalshear measurements and dynamic and static light scattering. By varying the polymer concentration, theeffect of entanglements on the amplitude and the dynamics of concentration fluctuations was studiedover a range of temperatures from the phase separation temperature (Tps) up to 30 deg above the θtemperature. While the time-averaged scattered intensity is not influenced by entanglements, dynamiclight scattering shows that the relaxation of the concentration fluctuations close to Tps is dominated bythe viscoelastic relaxation of the transient polymer network. The results are interpreted in terms of existingtheory on the influence of the elastic modulus on the relaxation of concentration fluctuations.