The accuracy of pyranometers used to measure diffuse and total solar radiation suffers from an error introduced by energy exchange with the inner dome. The Yankee Environmental Systems Isothermal Pyranometer employs a unique infrared sensing element and closed loop control system that virtually eliminates this error. The principle of operation of the Isothermal Pyranometer is given. This is followed by data obtained during the 2001 Diffuse Shortwave IOP held at the Atmospheric Radiation Monitoring site in Oklahoma. The instrument registered less than 1 Wm^-2 offset at night during the entire IOP. Nighttime data (offset) of the Isothermal Pyranometer plotted against net longwave radiation at the surface is centered about zero and shows no slope. Also, during the IOP an experiment was performed in which the shaded instrument domes were repeatedly capped and uncapped with an opaque hemisphere. The Isothermal Pyranometer output consistently fell to within 1 Wm^-2 of zero while capped. Results from other instruments that took part in the IOP are also shown for comparison.