The monolayer behavior at the air/water interface of the natural pigments hypocrellin A (HA), hypocrellin B (HB) and their butyl-amino derivatives (HAD, HBD) was investigated using Brewster angle microscopy (BAM). The results showed that stable monolayers were established with different phase transitions when the films were compressed. The collapse pressures of HA, HE, HAD and HBD were 30.3, 31.0, 34.8 and 35.5 mN m(-1), respectively which suggests that the butyl-amino substituents and the perylenoquinone rings of HAD and HBD are coplanar. The monolayer morphologies recorded by BAM showed differences among the images of these four compounds. Aggregation of HA, which may be attributed to the intermolecular hydrogen bonds between the two molecules, was observed. No obvious domains in the monolayers of HAD and HBD were observed using BAM, indicating that no large aggregations were formed in the monolayers.