Second-harmonic generation (SHG) is tested as an in situ technique for monitoring the properties of the Langmuir layer and the Langmuir-Blodgett deposition process. From compression-expansion of the Langmuir layer the SHG for a pure hemicyane dye increased sharply near a surface pressure of 20 mN m-1. For mixtures of hemicyanine with behenic acid the SHG increased sharply before any significant surface pressure was detected, indicating that the hemicyanine molecules are aligning at nearly zero surface pressure. Compression-expansion cycles of the Langmuir layer with hemicyanine suggest hemicyanine aggregate formation or two-dimensional crystal realignment. During Langmuir-Blodgett deposition the flow of the monolayer can be detected by both a surface pressure decrease and an SHG decreases.