Aliphatic epoxy composites with multifunctional polyhedral oligomeric silsesquioxane (POSS) ((C6H5CHCHO)(4)(Si8O12)(CH double bond CHC6H5)(4)) nanophases (epoxy/POSS 95/5 and 75/25) and epoxy blends with the prepolymer of ladderlike polyphenylsilsesquioxane (PPSQ) (95/5, 90/10, and 85/15) were prepared by solution casting and then curing. These composites and blends were studied by dynamic mechanical thermal analysis (DMTA) and mechanical testing. The POSS units incorporated into the epoxy network are well dispersed in the composite, probably on the molecular scale, even at high POSS content (25 wt %) based on TEM observations. However, the aliphatic epoxy/PPSQ blends exhibit good miscibility only at low PPSQ content (less than or equal to 10 wt %). Phase separation was clearly observed when the PPSQ content was 15%. Incorporation of the POSS macromer into this epoxy network by curing at upper temperatures of 120 and 150 degreesC broadened the temperature range of glass transition of the resulting composites but has almost no influence on their T-g (the tan delta peak temperature). The T-g of epoxy/PPSQ blends containing less than or equal to 10 wt % PPSQ increased slightly with increasing PPSQ content. However, the T-g of epoxy/PPSQ 85/15 is lower than that of the neat epoxy resin because cross-linking density is reduced in the blend. Inclusion of PPSQ into the epoxy resin has no effect on the width of their glass transition range. The storage moduli E' of both epoxy/POSS composites and epoxy/PPSQ blends at T > T-g are higher than those of neat epoxy resin and increase with the POSS or PPSQ content, improving their thermal dimensional stability. The flexural modulus of the epoxy resin is raised by POSS incorporation or PPSQ addition. Modification of the epoxy resin's flexural modulus is larger for composites with molecularly dispersed POSS than for those containing PPSQ. The magnitude of this increase goes up as more POSS or PPSQ was added. But, the flexural strengths of epoxy/POSS nanocomposites and epoxy/PPSQ blends are lower than that of neat epoxy.