Window solar gain is known to have a strong influence on building energy consumption and peak cooling load. Venetian blinds are routinely used to control solar gain. Software based on one-dimensional models is available to accurately predict the thermal performance of glazing systems, but the development of models for windows with shading devices is at a very early stage. A guarded heater plate apparatus has been used to measure center-glass heat transfer rates through a doubleglazed window with a venetian blind in the glazing cavity. Variables examined include pane spacing, temperature difference, slat angle, and the presence of a low-emissivity coating. Results were compared with earlier measurements. The data collected provide direct guidance in the development of models to predict U-factor and solar gain for this type of glazing/shading system, leading to a more structured and quantitative design procedure.