There exists in the literature many methods of determining the Angstr¿m parameters for the measurement of the optical thickness of aerosols. This thickness is required in cases such as when the spectral beam solar irradiance at ground level has to be assessed. The present work studies the differences between some of these methods and establishes the relationships between their results. A systematic study investigating 509 direct solar irradiance spectra was undertaken. As a result, a new wavelength pair for filter measurements is proposed to calculate the Angstr¿m parameters with the Voltz technic, namely, the pair (0.400 μm, 0.750 μm). This new pair is the one which best describes the solar beam attenuation in the atmospheric transmittance models. For the considered atmospheric conditions (continental urban aerosols), the mean value of the Angstr¿m parameter α is found to be 1.0¿0.3, which is relatively close to the conventional α=1.3 if taking into account the variance. The wavelength range explored is from 0.290 μm to 0.900 μm, which concerns many applications in bioscience.¿ 1997 American Geophysical Union