Sorbitol-type nucleating agents can be used to aid dispersion of polyhedral oligomeric silsesquioxane (POSS) molecules in isotactic polypropylene (iPP) and to promote self-assembly of POSS molecules into nanoparticles. In this paper, the interactions between a sorbitol-type nucleating agent and POSS molecules were investigated using polarized optical microscopy and rotational rheometry. Dibenzylidene sorbitol (DBS), a nucleating agent of iPP, was used in conjunction with two non-reactive POSS-trisilanolphenyl-POSS (ph-TPOSS) and octaisobutyl-POSS (bu-MPOSS). DBS formed three-dimensional fibrillar networks in iPP upon cooling from a homogeneous state. These fibrillar networks were characterized as gels using oscillatory shear rheometry. Trisilanolphenyl-POSS was incompatible with iPP but offered strong hydrogen bonding possibility with DBS; bu-MPOSS, on the other hand, was compatible with iPP but did not produce significant interactions with DBS. It was found that as low as 0.5 wt % of ph-TPOSS molecules in a ternary compound with 0.7 wt % DBS subdued fibrillar network formation by DBS and altered the rheological properties of the compounds. No such effect was seen in compounds of bu-MPOSS, although viscosity showed strong reduction. The storage modulus vs loss modulus data provided much better indication of changes of morphology in iPP/POSS and iPP/DBS/POSS compounds than loss tangent vs oscillatory frequency plots. (C) 2010 The Society of Rheology. [DOI: 10.1122/1.3439695]