Recent results from human imaging and electrophysiology promote the view that processing within auditory cortex can be influenced by cross-modal stimulation of other sensory modalities. Here we scrutinize the neuronal basis of these sensory interactions and ask whether they increase the information about the sensory environment available in neuronal responses. Recording field potentials and single unit responses in regions of the monkey auditory cortex we characterized the modulatory influence of visual stimuli on the responses to acoustic stimuli. While slow field potentials showed widespread visual modulations, only few individual neurons exhibited significant multisensory interactions, such as response enhancement or suppression. The visual modulation occurred only for a narrow time window of stimulus onset asynchronies and was independent of the particular kind of stimulus used. Using information theoretic analysis, we found that visual stimuli do not change the information in neuronal firing rates about the acoustic stimulus. However, visual modulation increased the information available in slow field potentials and in the phase of firing of individual neurons. Our results let us conclude that cross-modal input enhances the information available in auditory cortex about the environment and that this information is available not in the response strength of individual neurons but in a temporal neural code.