This work deals with the nitrates determination bj r the phenoldisulfonic acid method in the soils of São Paulo State. Several modifications were introduced which led to outline an analytical procedure. The nitrates are extracted by percolation, with a solution of H2SO4 0,01 N which is efficient in obtaining clear filtrates without clay and it is a saving time procedure. The ratio employed to remove soil nitrates is one weight of soil for three volumes of the extractor. The H2SO4 solution was the best extractor found because it dissolves less organic matter from the soil than other methods. Also, with the method described, the operation of removing chlorides is easy and it is possible to use a blank determination. Losses of nitrates are significant when chlorides are present in concentration as high as 11 e.μg (3.9 p.p.m.). The efficient removal of chlorides by using silver sulfate, can be performed only in acid extracts, if losses of nitrates are to be avoided. The solution free from chlorides must be neutralized with NaOH 1 N because acid reaction causes loss in nitrates which are liberated as HNO3 (gas). The residual organic matter is destructed with hydrogen peroxide after drying the solution. The hydrogen peroxide always has nitrates and the quantity of them must be determined. The residual salts from the above treatments are attacked with phenoldisulfonic acid and this solution dilutes to a convenient volume and neutralized with ammonium hidroxide (1 + 1). The color intensity developed in this reaction is read in an AC model Fisher Electrophotometer with B-425 filter calibrated for nitrates. The best readings are between 3-45 or 0,25-16,5 e.μg of NO-3.