In this article, we report on the viscoelastic and thermal properties of agarose-polyacrylamide (PAAm) interpenetrating polymer hydrogels (IPHs) and semi-IPHs as a function of agarose concentration and PAAm crosslinking degree. The results demonstrated that the agarose is able to gel in the presence of crosslinked and linear IPHs. In addition, the reticulation of PAAm in the presence of agarose is confirmed for the case of IPHs giving rise to systems with dimensional stability at high temperatures. The formation of a fully IPH was ascertained at low agarose concentrations. A study of the morphology and nanoscale elasticity of the different systems has been carried out with atomic force microscopy/ultrasonic force microscopy (UFM). UFM data provide further evidence of interpenetration, allowing us to visualize-if present-phase-separated domains with nanoscale resolution for the various crosslinking degrees and PAAm and agarose concentrations used during the formation of the IPHs. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2403-2412, 2010