For 14 headphones and 40 human subjects, transfer functions were measured from the voltage at the headphone terminals to the sound pressure at the entrance to the blocked ear canal. For all headphones the frequency responses were characterized by smooth fluctuations at low frequencies and rather individual high-Q resonances at high frequencies. Data were also obtained on the headphones' acoustical loading of the ear canal. The main objective of the investigation was an evaluation of the headphones as a means for the reproduction of binaural signals, but the transfer functions were also compared with design goals adequate for the reproduction of normal stereo recordings