Evolution of ferrofluid contained in two vacuum rotary feedthroughs is studied for the first time by a time‐of‐flight mass spectrometer in order to provide information on the background material these seals introduce into a high vacuum system. The seals used diester and poly(phenyl ether) based ferrofluids. The rate of evolution of the diester was found to be of the order of 3×10−10g/s and that of the poly(phenyl ether) was below the detection limit of the mass spectrometer, i.e., 10−10g/s. The oxygen and water evolution, which are generally of prime interest in vacuum application, were found to be 10−9g/s for both seals at 3,000 rpm. The feedthroughs were successfully employed in precision abrasion mass spectrometric experiments.