A radiotracer counting system was developed for adsorption and catalysis studies in ultrahigh vacuum using small area, single crystal surfaces. The counting system utilizes a rugged, compact, and rotatable surface barrier detector with a sensitivity sufficient to detect about lxlo 12 molecules containing carbon-14. The operating characteristics and performance of this counting system are discussed along with its application to studies of 14 c-benzene chemisorption and rehydrogenation on the (111) crystal face of platinum.