The DNA dodecamer of (CGCGmo6AATCCGCG) containing 2'-deoxy-N6-methoxyadenosine has been crystallized for X-ray analysis in order to investigate the effects of the modified adenosine on base pairing. It has been found that the crystal changes from one form to another during data collection in a manner similar to a phase transition. The two crystal structures show that this phenomenon, ascribed to differences in humidity, is correlated with a change in the contact angle between the two duplexes.