How can cognitive load in visual displays of computer simulations be optimized? Middle-school chemistry students (N = 257) learned with a simulation of the ideal gas law. Visual complexity was manipulated by separating the display of the simulations in two screens (low complexity) or presenting all information on one screen (high complexity). The mode of visual representation in the simulation was manipulated by presenting important information in symbolic form only (symbolic representations) or by adding iconic information to the display (iconic + symbolic representations), locating the sliders controlling the simulation separated from the simulation or integrating them, and graphing either only the most recent simulation result or showing all results taken. Separated screen displays and the use of optimized visual displays each promoted comprehension and transfer, especially for low prior-knowledge learners. An expertise reversal effect was found for learners' prior general science knowledge. Results indicate that intrinsic and extraneous cognitive load in visual displays can be manipulated and that learners' prior knowledge moderates the effectiveness of these load manipulations.