A three-wavelength sun-photometer was designed and built. The attenuation of direct solar light, at wavelengths of 0.38 micrometers , 0.50 micrometers and 1.00 micrometers were measured. Silicon photodiodes and interference filters of 10 nm FWHM were used. Aerosols were characterized by angstrom's turbidity formula in terms of coefficients (alpha) and (Beta) and by Moon's model in terms of the precipitable water content, w, and the amount of dust particles per unit volume, d. Aerosol attenuation was calculated from the slope of Langley's plots. Several measurements were performed in the city of San Luis during summer and autumn seasons of 1996. Results corresponded to a very clean atmosphere. Weather was extremely dry during most of measurements.