Solutions of polyisobutylene (PIB) in ethyl caprylate were investigated using mechanical shear measurements and dynamic and static light scattering. By varying the polymer concentration, the effect of entanglements on the amplitude and the dynamics of concentration fluctuations was studied over a range of temperatures from the phase separation temperature (T-ps) up to 30 deg above the 0 temperature. Virile the time-averaged scattered intensity is not influenced by entanglements, dynamic light scattering shows that the relaxation of the concentration fluctuations close to T-ps is dominated by the viscoelastic relaxation of the transient polymer network. The results are interpreted in terms of existing theory on the influence of the elastic modulus on the relaxation of concentration fluctuations.