A new modular Active Cavity Radiometer Irradiance Monitor (ACRIM) has been developed for the Earth Observation System (EOS) ACRIM III experiment. ACRIM III will be launched on a dedicated small satellite (ACRIMSAT) in late 1999 for a five year EOS Phase I total solar irradiance (TSI) monitoring mission. The new ACRIm instrument is compact, reduced in mass and size to less than 1/2 that of previous ACRIM's. The enhanced structure, electronics and shutter system, designed collaboratively by the Columbia-based Principal Investigator and Jet Propulsion Laboratory instrumentation teams, provide a versatile, precise and accurate, state-of-the-art TSI observational capability for the EOS and other potential applications.