Mixed insoluble monolayers of bis(N-ethy,N-octadecyl)rhodamine perchlorate (RhC18) and arachidic acid (ArAc) at the air-water interface were characterized by surface pressure-area isotherm, second harmonic generation (SHG), and reflection spectroscopy studies. The analysis of surface pressure-area isotherms of mixed RhC18/ArAc layers provided evidence that there was a high degree of miscibility of film components and a strong interaction among them. A new band was found to appear on the blue side of the reflection spectrum after an increase in both the concentration of ArAc in the layer and the applied surface pressure. This new band was assigned to dye aggregates, where spontaneous formation in the gaslike region was promoted by the presence of ArAc and induced by compression in the region of the continuous monolayer. Both in gaslike and in compressed film, the formation of blue-shifted aggregates coincided with the increase of the SHG response of the monolayer. Resonance SHG enhancement was proven to be the main reason for this observation due to electronic resonance of aggregates with the incident light frequency.