Based on cognitive load theory, two experiments investigated the conditions under which audiovisual-based instruction may be an effective or an ineffective instructional technique. Results from Experiment 1 indicated that visual with audio presentations were superior to equivalent visual-only presentations. In this experiment, neither the auditory nor the visual material could be understood in isolation. Both sources of information were interrelated and were essential to render the material intelligible. In contrast, Experiment 2 demonstrated that a non-essential explanatory text, presented aurally with similar written text contained in a diagram, hindered learning. This result was obtained because when compared to a diagram only format, the aural material was unnecessary and therefore created a redundancy effect. Differences between groups were stronger when information was high in complexity. It was concluded that the effectiveness of multimedia instruction depends very much on how and when auditory information is used.