Infrared spectra of aqueous antibiotic solutions were obtained using a new Liquid Analyzer accessory in a Fourier transform infrared (FT-IR) spectrometer. Based on a cylindrical internal reflectance element, the Liquid Analyzer accessory provides a sufficiently short (∼0.015 mm) effective pathlength for infrared spectroscopy of aqueous solutions from 3200 to 800 cm−1, Spectra of aqueous solutions of penicillin VK, sodium oxacillin and sodium methicillin are shown. By measuring the absorbante of the beta-lactam carbonyl band, FT-IR spectroscopy with the accessory exhibits analytical sensitivity to less than 0.1% concentration by weight of antibiotic both in aqueous solution and in an actual fermentation broth.