We have studied twinning in GaAs nanowires grown via holes in a silica mask deposited on GaAs(111)B substrates by metal‐organic chemical vapour epitaxy without catalysts. Twins perpendicular to the growth direction form in the nanowires, their {111}‐type side facets leading to corrugation of their {110} side walls. The top facets are almost atomically smooth. Aberration corrected annular dark‐field scanning transmission electron microscopy reveals all twins are rotational, commencing with layers of Ga and finishing with As atoms. Energy‐loss spectroscopic profiling has shown no significant changes in the band‐gap or bulk plasmon energy at those twin boundaries, and the observed reduction of the interface plasmon energy by ∼0.13 eV is close to the detection limit of the technique, reflecting the very low energetic electronic changes related to twin formation. We studied twinning in GaAs nanowires grown via holes in a silica mask deposited on GaAs(111)B substrates by metal‐organic chemical vapour epitaxy without catalysts. {111} twins perpendicular to the growth direction lead to corrugation of their {110} side walls. All twins are rotational, commencing with Ga and finishing with As atoms. Energy‐loss spectroscopic profiling showed no significant changes in the band‐gap or plasmon energy at those twin boundaries, reflecting the small energetic changes related to twin formation.