We give an account of X-ray diffuse scattering (XDS) as a non-destructive technique to characterize the defect structure of organic multilayers. As well as the interface roughness we especially investigated the lateral correlation length tau of scattering aggregates using the angle-dispersive as well as the energy-dispersive scattering from a Pb stearate film. Although the present theoretical model is not satisfactory, it is possible to relate tau to the in-plane dimension of the microcrystalline domains. The XDS and the X-ray specular reflectivity (XSR) depend in a different manner on the interface roughness and the domain size. Therefore the ratio of XDS and XSR measured at special points of reciprocal space can be used to observe the changes in these real structure parameters. This is demonstrated in the case of thermally induced melting of domains.