Ultra small-angle neutron scattering (USANS) methods and equilibrium swelling measurements are combined to characterize correlation length, pore volume fraction, and gel-phase water content in sponge-like hydrogels. USANS invariant analysis is applied to poly(hydroxyethylmethacrylate) gels prepared with 40-80 vol. % of an extractable porogen. These gels are strongly opaque in the swollen state, but become transparent upon drying due to shrinkage or disappearance of the pores. The correlation length associated with porosity is measured by fitting USANS data to an appropriate scattering model. By combining swelling measurements with USANS invariant analysis, it is possible to determine the equilibrium volume fraction of pores and the concentration of water in the gel phase without performing contrast variation measurements. (C) 2010 Elsevier Ltd. All rights reserved.