The lamellar structure of a nearly symmetric diblock copolymer in the weak segregation regime is investigated using small-angle X-ray scattering (SAXS). The scattering curves measured at room temperature are analyzed using the concept of the interface distribution functions. The determination of the interfacial thickness shows that an additional component of the scattering function has to be taken into account to obtain values comparable to theoretical ones. This component which has so far been neglected in similar studies is due to the statistical structure of the domain boundary. The temperature dependence of the interfacial thickness and of the lattice constant, obtained using synchrotron SAXS from room temperature up to the microphase separation transition (MST), is in fair agreement with recently published theories.