The surface pressure PI-residual area A and the surface potential DELTAV-residual area A isotherms of poly-gamma-benzyl-L-glutamate (PBLG) in the alpha-helical conformation have been studied at the air water interface. We observe in the PI-A isotherm the monolayer-to-bilayer transition already reported in the literature. At large residual areas, DELTAV is found to fluctuate, indicating that the PBLG molecules form aggregates. Indeed, these aggregates have been evidenced by epifluorescence microscopy. To gain further insights, we calculated the corresponding perpendicular dipolar moment mu(perpendicular-to).mu(perpendicular-to) decreases linearly with the decreasing apparent residual area from the onset of the transition. This is inconsistent with the fact that, during the transition, the surface area of the residues in contact with water should stay constant and accordingly the real mu(perpendicular-to) should slay constant. In addition, immediately before the onset of the transition, the calculated mu(perpendicular-to) values are found to be overestimated. We show that this fact results from an overestimation of the real surface area of the PBLG residues as a consequence of the existence of aggregates in this region. Finally. we propose that in such aggregates the surface area of PLBG residues must be similar to that found at the onset of the transition.