Knowledge about the critical interfacial concentration of a protein supports our understanding of the kinetic stability of an emulsion. Its determination is currently limited to either invasive or indirect methods. The aim of our study was the determination of the critical interfacial concentration of whey protein β-lactoglobulin at oil/water-interfaces through fluorescence and pendant drop analysis and the comparison to an in situ Fourier-transform-infrared-spectroscopy (FTIR) method. Exponentially decreasing interfacial tension with increasing β-lactoglobulin content (0.10–1.00 wt%) in pendant drop analysis could partly be confirmed by fluorescence spectra. A critical interfacial concentration of 0.20–0.31 wt% β-lactoglobulin (1.80–2.69 mg/m²) in oil/water (5/95)-emulsions was determined via FTIR, analyzing the Amide I/Amide II peak intensity ratio. This was confirmed by the increasing formation of intermolecular β-sheets, revealed by second derivative spectra. With this FTIR method we expand current options to investigate the interfacial behavior of food proteins by determination of secondary structure elements.