A search method for investigating the extent to which chloro-methyl interchange is exhibited by crystal structures deposited with the Cambridge Crystallographic Database is presented. Using the October 1998 release with 190,307 data entries, 105 pairs were identified for which there was a common molecular framework, differing only in the presence of a chloro or methyl group. Approximately 30% appear to demonstrate isotructural packing arrangements. Two examples within the database that were found nor to show CI-Me interchange were subsequently examined from the viewpoint of structural mimicry and the formation of mixed crystals.