A series of experiments measured the audiovisual stimulus onset asynchrony (SOA_AV), yielding facilitative multisensory integration. We evaluated (1) the range of SOA_AV over which facilitation occurred when unisensory stimuli were weak; (2) whether the range of SOA_AV producing facilitation supported the hypothesis that physiological simultaneity of unisensory activity governs multisensory facilitation; and (3) whether AV multisensory facilitation depended on relative stimulus intensity. We compared response-time distributions to unisensory auditory (A) and visual (V) stimuli with those to AV stimuli over a wide range (300 and 20 ms increments) of SOA_AV, across four conditions of varying stimulus intensity. In condition 1, the intensity of unisensory stimuli was adjusted such that d′ ≈ 2. In condition 2, V stimulus intensity was increased (d′ > 4), while A stimulus intensity was as in condition 1. In condition 3, A stimulus intensity was increased (d′ > 4) while V stimulus intensity was as in condition 1. In condition 4, both A and V stimulus intensities were increased to clearly suprathreshold levels (d′ > 4). Across all conditions of stimulus intensity, significant multisensory facilitation occurred exclusively for simultaneously presented A and V stimuli. In addition, facilitation increased as stimulus intensity increased, in disagreement with inverse effectiveness. These results indicate that the requirements for facilitative multisensory integration include both physical and physiological simultaneity.