Photon correlation spectroscopy and pulsed-field gradient nuclear magnetic resonance have been utilized in order to investigate the characteristic features of the recently established (both experimentally and theoretically) diffusive "polydispersity" relaxation process far concentration fluctuations in homogeneous diblock copolymer melts and solutions. This is accomplished using semidilute solutions of a symmetric mixture of two diblock copolymers with similar molecular weights and almost mirror compositions in a common good solvent. Mixing of the two asymmetric diblocks leads to a system with almost symmetric composition and with narrow molecular weight distribution, but with large effective composition polydispersity. Above the order to disorder transition (ODT), the theoretical expressions for the amplitude and the relaxation rate of the polydispersity mode can quantitatively describe the observed diffusive relaxation. Similarly to the situation in homopolymer blends, the thermodynamic forces can significantly retard the diffusion coefficient and lead to an increase in the dynamic intensity with increasing copolymer concentration. The intervention of the ODT alters this concentration dependence at copolymer concentrations phi near but below phi(ODT).