We investigate the effect upon HREM images of the static atomic displa
cements (SD) present in crystalline alloys with atomic size effect. Ou
r method is to compare pairs of images of the same distributions of at
oms (in III-V alloys) calculated with and without taking the SD into a
ccount. We show that large differences usually exist between these ima
ges, assess them quantitatively, and demonstrate that proper considera
tion of the SD is essential to achieve reliable composition analysis b
y HREM, in particular when the 'chemical mapping' technique is used. T
he effect of the SD depends on the specimen thickness and on the objec
tive lens defocus.