A novel technique that links vegetation properties and ET fluxes with a microwave "emissivity difference vegetation index" (EDVI) has been developed and applied to the Amazon region. These EDVI values can be derived from a combination of satellite microwave measurements with visible and infrared observations. This technique is applicable both day and night times under all-weather conditions, which is particularly important for remote sensing since under cloudy conditions classic optical techniques are not applicable. For the Amazon basin, EDVI captures vegetation variation from dense vegetation (rain-forest) to short and/or sparse vegetation (savanna) under all-weather conditions. Good relations between microwave based EDVI and optical indexes of NDVI and EVI are found for various vegetation conditions. More importantly, EDVI shows no sign of saturation even for the tropical rain forest, while NDVI (and EVI to a lesser extent) is clearly saturated. Over the Amazon region in a normal dry season day, EDVI can provide the vegetation information over 98% of the land surface while the optical vegetation indexes can be retrieved only for a small fraction (14%) of the region.