Using in situ optical microscope observations of the surface of polyphenylene oxide (PPO)/chloroform solution in the breath figure (BF) process, the detailed formation phenomenon of surface of the BF pattern was obtained. This phenomenon directly supports the "bursting hypothesis". Based on the "bursting hypothesis" and additional experimental data, the mechanism for the late BF process of the PPO/chloroform system is discussed. Moreover, the formation process of defects on the film surface was also directly observed, and the defect formation mechanism of the system is discussed. This work can help to further understand the formation of patterns and defects on the surface of BF films. (C) 2010 Elsevier Ltd. All rights reserved.