The vectorial orientation of the peptide ZnPPM-BBC16 in a Langmuir monolayer was studied by means of X-ray reflectivity and polarized epifluorescence. ZnPPM-BBC16 consists of two alpha -helical 31-mers dimerized via a disulfide bridge between two N-terminal cysteines (alpha -S-S-alpha) so that two bis-his metalloporphyrin binding sites between positions 10,10' and 24,24' are formed. Zn(II) protoporphyrin M was bound to the position 24. To enhance stability at the interface, a palmitoyl (C16) chain was bonded to each N-terminal cysteine. X-ray reflectivity measurements make it possible to infer the orientation of the alpha -helices by determining the electron density profile of the monolayer. Polarized epifluorescence provides the orientation distribution of the porphyrin with respect to the monolayer plane. Both X-ray and fluorescence measurements show that at low surface pressure (pi = 5 mN/m), the peptide alpha -helices lie in the plane of the water surface with a narrow orientational distribution of the porphyrin. At high pressures (pi > 30 mN/m), the peptide alpha -helices are perpendicular to the water surface and the distribution is wider. At intermediate pressures the peptide alpha -helices in the monolayer undergo a transition from the parallel to perpendicular orientation with almost a complete loss of the orientational order of the porphyrin.