Pseudo interpenetrating polymer networks were prepared by a sequential method using a high molecular weight natural rubber and poly(methyl methacrylate), PMMA. The thermal properties and morphology were investigated varying the average molar mass between crosslinks ((M) over bar(c)) and the composition. The extent of crosslinking of natural rubber plays a major role in the transitions domains whatever the sample composition. As the crosslink density decreases, only one T-g value is observed ((M) over bar(c) of 500 and 1000 g/mol). Comparison between the pseudo IPNs at different (M) over bar(c) values and compositions indicates that there are no significant differences in the thermal stabilities of the materials. Crosslinking and the component composition had a strong effect on the morphological patterns of the samples. For a higher crosslink density ((M) over bar(c) = 142 g/mol), close and compact phase domains can be seen. The morphologies for lower crosslink densities ((M) over bar(c) of 500 and 1000 g/mol) showed fibrillar domains characteristic of the PMMA.