In clay nanocomposites, the specific interface area is the key factor determining potential improvements of properties. Nevertheless, in most systematic studies of nanocomposites little emphasis is put on assuring and characterizing dispersion quality. To probe the influence of dispersion quality, we compare nanocomposites filled with two layered silicates which were made by melt compounding and solution blending, respectively. Poly(methyl methacrylate) (PMMA) is chosen here as a thermoplastic model matrix which was compounded with a synthetic nano-mica (O-hect) and commercial Bentone with typical diameters of 5-7 mu m and <300 nm, respectively. The dispersion quality was monitored by mu-computer tomography (mu-CT) and transmission electron microscopy (TEM). Moreover, gas barrier measurements proved to be an additional independent and very sensitive probe. Reductions of the oxygen permeation at similar to 4 wt.% by 60% and 30% for solution blended and melt compounded samples directly evidence a mediocre dispersion in the latter. Structure-property relationships were established by in-depth mechanical testing and the properties were correlated with the improved morphology. Significantly higher stiffness was achieved by enhanced dispersion quality for O-hect-filled nanocomposites without causing any embrittlement. Interestingly, a maximum increase of fracture toughness (63%) was obtained at filler content as low as 0.8 vol.% for the solution blended sample. A similar improvement of fracture toughness for the melt compounded sample afforded more than twice the clay content emphasising the crucial influence of specific surface area. This highlights the importance of the solution blending method to exploit the full potential of nanofillers and suggests that only samples with comparable dispersion quality may be compared. (C) 2014 Elsevier Ltd. All rights reserved.