Three kinds of intensity-difference thresholds were compared for the purpose of determining interaural-binaural and interaural-monaural relations. Five subjects were tested at 250, 2000, and 6000 Hz, using a variation of the Békésy technique. Response to 27 interaural intensity combinations were measured at each frequency. Monaural and binaural conditions were included. Each of 24 combinations was measured under 0° and 180° interaural phase conditions, and all were measured at 50, 35, and 20 dB HL (ISO). Results indicate that interaural-difference thresholds decrease as a function of level, as might be predicted. These thresholds vary as a function of the frequency and apparent azimuth of the fused tone, but are relatively insensitive to variations in interaural phase. Interaural intensity-difference thresholds differ significantly from both monaural and binaural difference thresholds.