Lipases are enzymes classified as serine hydrolases (EC 3.1.1.3), being soluble in water. They hydrolyze ester bonds in triglycerides, but catalyze also esterification and transesterification, acting only at the interface water/oil.  Lipases are widespread among natural sources such as microorganisms, animals and plants (fruits, reserve tissues and mainly seeds), finding promising applications in chemical processes, detergents, organic synthesis, and waste treatment. The present work intended to establish a protocol for soybean oil hydrolysis, that constitutes a common residue discarded incorrectly in the drain after frying, probably by most users. The enzyme was submitted to extraction, clarification and concentration, using dithiotreitol and sodium metabisulfite as antioxidants. A standard curve of hydrolysis was obtained using different enzyme concentrations, up to 10mg/mL. The enzyme assay was carried out at 37ºC for 60 minutes, and the activity was measured by titration with 50mM KOH, using phenolphthalein as pH indicator. For optimal pH the assays were performed in the range from 3.0 to 10.0, using 50% soybean oil, 10,17% Triton X-100, 0.002%  CaCl₂ (10mM)   33.3% of suitable buffers and 0.0098% of crude extract (5mM). The solution was titrated as described above. We also performed tests involving varying the exposition time of the oil to the enzyme, up to 2 hours. The data showed that a 5mM lipase concentration is capable of performing a significant hydrolysis. Optimal pH is alkaline, around 8.0-8.5, and the exposition time was around 90 minutes. Financial support: FAPESP 05/03157-1 (FDAF), CAPES (PPP), FAPESP 03/00085-4 and CNPq (GOBR).