The free convective heat transfer in a double-glazed window with between-panes Venetian blinds was measured using a Mach-Zehnder interferometer. A vertical cavity with differentially heated/cooled flat plates was set up with an internal blind at slat angles of ø=0⁰, ø=45⁰, and ø=90⁰ from the horizontal and tip-to-plate spacings of s=2mm, s=4mm, and s=8mm. Heat transfer measurements were taken with air as the test fluid and at Rayleigh numbers of Ra~4.5x10(4), RA~6.7X10(4), and Ra~13.1x10(4), based on cavity widths of W=28.7mm, W=32.7mm, and W=40.7mm, respectively. Finite fringe interferograms were used to obtain local and average heat transfer data. Infinite fringe interferograms were taken to visualize the temperature field within the cavity. A preliminary numerical study of the experimental geometry was also conducted. The results show that there was substantial variation in local heat transfer rates caused by the presence of the between-panes blind inside the window cavity. In general, experimental average Nusselt numbers were found to be lower than those of a cavity without blinds.