The onset of twinning from ((2) over bar(1) over bar(1) over bar) to ((1) over bar(3) over bar(3) over bar) in large volume Cd1-xZnxTe crystals, grown by vapour transport on ((2) over bar(1) over bar(1) over bar), often referred to as (211)B, oriented GaAs seeds, has been investigated using X-ray diffraction imaging (X-ray topography). Twinning is not associated with strains at the GaAs/CdTe interface as the initial growth was always in ((2) over bar(1) over bar(1) over bar) orientation. Nor is twinning related to lattice strains associated with injection of Zn subsequent to initial nucleation and growth of pure CdTe as in both cases twinning occurred after growth of several mm length of Cd1-xZnxTe. While in both cases examined, there was a region of disturbed growth prior to the twinning transition, in neither crystal does this strain appear to have nucleated the twinning process. In both cases, un-twinned material remained after twinning was observed, the scale of the resulting twin boundaries being sub-micron. Simultaneous twinning across the whole sample surface was observed in one sample, whereas in the other, twinning was nucleated at different points and times in the growth. (C) 2016 Elsevier B.V. All rights reserved.