Surface potential measurements have been used to determine the dipole interactions of the acetylcholinesterase (AChE) monolayer at the air/aqueous interface. The results show that it is the ionogenic groups of the enzyme that cause significant changes in the surface potential of the monolayer as the pH of the sub-phase is changed. The atomic force microscope (AFM) images of AChE Langmuir-Blodgett (LB) films indicate that at low pH, an unfolding of the enzyme occurs which leads to formation of large domains at the air/aqueous interface rather than an organized AChE monolayer. Furthermore, the AFM images of the AChE LB film prepared at basic pH show that the enzyme forms aggregates.