The synthesis of poly(ethylmethylsiloxane-block-dimethylsiloxane), EM, copolymers with two blocks that possess similar segmental mobilities, low glass transition temperatures, negligibly small optical anisotropies, but sufficiently different refractive indices was performed in order to study the dynamics of composition fluctuations in disordered diblock copolymer melts. Photon correlation spectroscopy was employed to measure the composition correlation functions C(q,t) for six EM samples with total degree of polymerization N between 240 and 1200 at different wave vectors q and temperatures. For all samples but the one with the lowest N, C(q,t) was found to display two distinct relaxation processes with characteristic pertinent features, in addition to the long range density fluctuations. For the fast relaxation mode, the q-independent relaxation decay rate GAMMA1 varies with N-3, whereas the N-dependent amplitude S1(q) exhibits a q2 dependence. In the low q limit, the predictions of the random phase approximation for the internal relaxation of the copolymer chain conform well to these experimental findings and, moreover, account almost quantitatively for the values of GAMMA1, and S1(q) of the symmetric EM’s. The second relaxation process has a diffusive (q2-dependent rate) character with q-independent amplitude S2(q) and its diffusion coefficient D, agrees well with the self diffusion of the copolymer chain either derived by shear viscosity data or measured by pulsed-field gradient NMR. This second process can be attributed to additional concentration fluctuations due to the inherent composition polydispersity, and, hence, S2(q) is a measure of its degree, whereas D is the mutual diffusion coefficient. In the EM samples, the latter is predicted and found to be very similar to the self-diffusion coefficient.