Neutron and X-ray scattering studies of Bi2Sr1.9La0.1CuO6+δ and Bi2Sr2PrCu2O8+δ single crystals have revealed the existence of diffuse scattering streaks associated with two-dimensional structural fluctuations which are temperature dependent. By means of heat treatment, one can preserve or destroy the three-dimensional coupling. We believe that this effect is related to the local order of the oxygen atoms in the BiO layers which controles, at least partly, the charge transfer to the CuO2 layers.