The headgroup orientation of a merocyanine dye, 1-docosyl-4-(4-hydroxystyryl)pyridinium bromide, has been studied as a function of surface pressure on an acidified and a buffered subphase at pH 4 by second harmonic generation (SHG). The protonated form of the dye is SHG active for a fundamental wavelength of 800 nm. Phase transitions observed in the pressure-area isotherms correlate with changes in headgroup orientation, thus yielding microscopic information on the nature of the surface phase transition. The polarization resolved SHG data are analyzed to recover the orientation of the merocyanine headgroup as a function of surface pressure. The complex index of refraction of the surface film has been taken into account. Both the measured pressure-area isotherms and the orientation of the headgroup are found to be dependent on the nature of the subphase. In particular, at low pressure the acidified subphase yields a more vertical mean orientation (24 degrees) of the headgroup than the buffered surface (42 degrees), while at high pressures, above the expanded-to-condensed phase transition, the mean orientation of the headgroup is nearly independent of subphase (> 50 degrees). This behavior is discussed in terms of complex formation between the headgroup and the phthalate ions in the buffered subphase and shows how complexation of the adsorbate from the subphase can influence adsorbate orientation.