This paper describes fourier transform infrared spectroscopy used by the Bureau of Mines to study, in situ, the reaction of aqueous solutions of potassium ethylxanthate (KETX) with copper-activated sphalerite. The use of a transmittance technique is possible for sphalerite because, besides being the reactive mineral surface, sphalerite is also an infrared-transparent material in the region where some of the characteristic vibrational bands of xanthates occur. Bands characteristic of copper(I) ethylxanthate were evident after 5 min of reaction between aqueous KETX solutions and activated plates. The bands continued to grow as the reaction proceeded and were also present on dried surfaces after the reaction. Diethyldixanthogen dissociated on copper-activated surfaces, and bands characteristic of copper(I) ethylxanthate were detected on the surface of dried plates. No evidence for zinc(II) ethylxanthate formation with either unactivated or activated plates was obtained.