A definition of binaural additivity is given in terms of the theory of simultaneous conjoint measurement. Additivity is then tested and verified by a conjoint measurement procedure. Methods for deriving psychophysical scales from such procedures are discussed, and the experimental scales are compared with the usual ratio scales for loudness, derived from extensive measurement such as magnitude estimation. The functions are in good agreement and it is concluded that binaural additivity of loudness holds for non‐zero stimulation of the ear