Poor understanding of the effects of aerosols on climate is one of the most significant factors that limits more accurate quantification of regional and global climate change. Comprehensive and accurate observations of aerosol chemical and physical properties and their direct and indirect radiative forcing are needed to provide sufficient constraints on the problem for more thorough assessment by numerical models. This study reports on a newly established site in Southwestern Puerto Rico designed to provide long-term and accurate measurements of aerosols. Specifically, results are reported on aerosol optical properties derived from a Multifilter Rotating Shadow-band Radiometer (MFRSR) that was installed in June, 2005 at the Magueyes Island in Southwestern Puerto Rico (latitude = 17º 58' 13.5” N, longitude = 67º 2' 43.5” W, altitude = 12 m) as part of the NOAA Center for Atmospheric Sciences (NCAS) MFRSR based aerosol optical depth and radiation network (AERADNET). Retrieval algorithms were derived to obtain one-minute averages of aerosol optical depth and Ångström exponent from irradiance measurements under clear sky conditions. Our analysis was focused at 496, 612, 670 and 868 nm, with a total of 59 clear days. Seasonal variations of AOD values with maximum values during the summer (0.62) and minimum values during winter (0.03) were found. Ångström exponent (α) results (between 496.2 nm and 868.3 nm) were as low as 0.10 during the most severe dust events and as high as 1.45 in association with finer particles from local sources. A comparison of AOD and Ångström exponent (α) between the MFRSR and a collocated AERONET station shows remarkable agreement between both instruments. Correlations coefficients of 0.99, 0.99 and 0.92 were determined for TOD at 500, 675, 870 nm respectively, 0.97, 0.98 and 0.92 for AOD at 500, 675, 870 nm and 0.94 for α between 500 nm and 870 nm. Comparison between MFRSR and SeaWiFS satellite sensor was also performed with correlation coefficients of 0.96. As a final analysis a classification of aerosols for selected cases is also presented.