The internal structure of thermoreversible poly(N-vinyleaprolactam-co-N-vinylpyrrolidone) microgels has been studied by static and dynamic light scattering (SLS/DLS) and small-angle neutron scattering (SANS). A dramatic decrease in the radius of gyration R-g and the hydrodynamic radius R-h, is observed during the phase transition near 30 degrees C. At 15 and 30 degrees C the experimental SLS/SANS curves can be fitted to a model of polydisperse spheres with rough surfaces. The resulting fit parameters are discussed in the framework of changes in the internal structure of the particles during the phase transition. In the completely collapsed state at 50 degrees C, the scattering response can be described by a modified Pored behavior.