Small-angle X-ray scattering (SAXS) is employed to study the density and concentration fluctuations near the microphase separation in poly(styrene-b-phenylmethylsiloxane) copolymers. The former were calculated from the measured isothermal compressibility and are continuous throughout the transition. The residual SAXS background intensity due to local concentration fluctuations is a sensitive probe of the order-disorder transition. The same results have been obtained by independent computer simulations of dense diblock copolymer melts.