In order to determine which component contributes most to the vertical attenuation coefficient for downward irradiance [Kd(PAR)] in Ríogrande II reservoir, a multiple linear regression model was used to obtain Kd(PAR) considering the sum of partial Kd contributed by water (Kw), chlorophyll a (Kchl a), particulated inanimate material (tripton, Ktripton), and phytoplankton-like particles (Kp-phyto). Samples of water were taken with a Schindler bottle. The photosyntetic active radiation (PAR) was measured with a Li-cor quantameter at five sampling stations every 10 days from July 2002 to July 2003. Total suspended solids, inorganic suspended solids, volatile suspended solids, and chlorophyll a were quantified. Given that the Ktripton (1,14 m-1) value was the highest, the prediction that the influence of Kchl. a, Kp-phyto and Ktripton would be high and similar at the upstream Chico River station was partially accepted. The equation to estimate Kd(PAR) for the reservoir was Kd(PAR) = 1,32Dsd -0,93, with Dsd explaining 62,4 % of the variation in Kd . Optically, the reservoir was classified as T-type and case 2, which correspond to a turbid system where despite its eutrophic condition and high productivity, tripton and not phytoplankton was the fraction that harvested more photosynthetic quanta.