The chlorophyll-a specific absorption coefficient (aphy*) is used to describe phytoplankton light absorption capability per unit of chlorophyll-a (chl-a). This coefficient has an important role in remote estimate of chl-a concentration and primary productivity, because when treated as a constant it can be a significant source of uncertainty in these models. aphy * (lbd) could vary both seasonally and spatially, because it is dependent on the physiology and ecology of phytoplankton communities, which vary with external factors such as turbulence induced in the water column by the increase of discharge peaks in reservoirs. The aim of this study is to analyze the spatial-seasonal variability of aphy* (lbd) in Ibitinga/SP Reservoir, in order to better understand the dynamic of light absorption by phytoplankton in optically complex waters. The methodology is based on laboratory remote sensing for measuring the phytoplankton absorption coefficient (aphy (lbd)) by means of a spectrophotometer according to Tassan and Ferrari (2002) method. The aphy* (lbd) was obtained dividing aphy (lbd) by chl-a concentration determined according to Nush (1980). The highest amplitude chl-a concentration, for the period of field campaigns, occurred in the months of February and September. Analyzing the aph* (lbd), it was noted that the spectrum had a higher variability in magnitude than in shape, especially in the blue and red region, both spatially as seasonally.