Neutron specular reflection has been used to probe the structure of monolayer films of an anilino squaraine, 2,4-bis(4-(N-methyl-N-octylamino)phenyl) at the air-water interface. The thickness and area per molecule indicate that the chromophore aligns with its long axis parallel to the water surface in the low-pressure regime (l = 15.2 +/- 0.5 Angstrom and A = 87 +/- 4 Angstrom(2) molecule(-1) at pi = 7 mN m(-1)) but tilts upward when the layer is compressed (l = 17.5 +/- 0.5 Angstrom and A = 72 +/- 3 Angstrom(2) molecule(-1) at 18 mN m(-1)). The absorption spectra of the Langmuir-Blodgett (LB) films are dependent upon the deposition pressure and indicate different aggregation phenomena : purple (lambda(max) = 545-555 nm and 660-670 nm) and blue-green (lambda(max) = 660-690 nm) phases exhibit strong second-harmonic generation (SHG) with effective second-order susceptibilities of 20-40 and 30-60 pm V-1 respectively at 1.064 mu m. Spun-coated films are also SHG-active, and this is attributed to a partially aligned mosaic of acentric aggregates in films of the pure dye and a distribution of aggregates when dispersed in poly(vinyl acetate). Electrospray ionization mass spectrometry has provided evidence of aggregation in solution with masses which correspond to the monomer (517, [M + H](+)) and dimer (1033, [2M + H](+)). The aggregate, rather than the centric monomer, is responsible for the anomalous second-order nonlinear optical properties.