General Belgrano reservoir, in a subtropical region of the province of Salta, is one of the most important in Argentina because of its size and purposes. Previous studies indicate a high speed of increasing its trophic state, adversely affecting the quality of the resource. This paper characterizes the structure (grain size, organic matter content) and amount of phosphorus fractions in sediments, to establish its dynamics during the transition from stratification until mix period. Physical-chemical major variables (temperature, pH, redox potential, conductivity) and phosphorus content (total, reactive soluble and organic) were analyzed according to standardized techniques, at three Secchi depths, interfase and sediment, from March to July, 2011. Water quality in all sites showed high uniformity: it was slightly basic with high conductivity and low concentration of bioavailable phosphorus indicating the extreme mobility of the nutrient in the food chain. The sediments, whose characteristics differed significantly from the overhead compartments, were neutral, with highest conductivity and redox potential values indicating anoxia throughout the study. The tributaries zones have clayey composition, while the lentic area is sandy, which is related to the raised content of phosphorus adsorbed in the first ones. The great amount of phosphorus in sediments demonstrated its relevance as a source of internal eutrophication of the reservoir.