Single-phase icosahedral Al 7 0 . 5 Pd 2 1 (Re,Mn) 8 . 5 and Al 6 5 Cu 2 0 Os 1 5 samples have been studied in order to find a relationship between electronic localization and structural properties of icosahedral quasicrystals. Annealed at high temperatures (HT), e.g. 940 o C, the Al-Pd-Re samples exhibit a behavior which can be described as nearly localized, while annealed at low temperatures (LT), e.g. 600-640 o C, localization has been observed. Similar observations have been made for icosahedral Al-Cu-Os. Accordingly, Al-Pd-Re and Al-Cu-Os can be considered as model systems for this type of investigation. Performing transmission electron microscopy measurements, we have observed and quantified the phason strain in these samples: For the two model systems the HT-states are closer to a perfect icosahedral state containing some random phason strain, in the LT-states we observed a strongly phason-perturbed i-phase. Additionally, the influence of chemical disorder was studied by the addition of Mn into icosahedral Al-Pd-Re.