Fatty spew is one of the most frequent defects appearing on finished leathers, especially sheepskins. Origins of this problem include an insufficient degreasing process, presence of excess residual natural fat in leather, quantity and quality of fatliquoring material added during the fatliquoring process and defective fatliquoring techniques. It is usually not easy to identify this problem during the processes. The aim of this study is to find out the limit value of natural fat that cause to fatty spew and its effect on leather quality. In the experiments Irish ovine sheepskins were used, as they are known for their high natural fat content. The experiments were carried out in three groups. The first group was processed without degreasing and fatliquoring. The second group was degreased and processed without fatliquoring and the third group was degreased; and fatliquored with 3, 5 and 7% of their own emulsified fat. The application of 3% fat emulsion is agreeable due to fatty spew formation in 5% and 7% fat emulsion application, as expected from preliminary tests. Some physical and chemical characteristics of natural and emulsified fats were investigated such as density, acid, peroxide and iodine values. Visual images of skin surfaces were obtained by Scanning Electron Microscopy (SEM). Gas Chromatography (GC), Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared Spectroscopy (FT-IR). Analyses were performed for the determination of fatty acids components that cause fatty spew defect. It was determined that double bond between C and C in the structure of the palmitoleic and linoleic acids was saturated, the ratio of palmitic and stearic acids was increased and palmitic and stearic acids were found to be the main reasons for fatty spew formation.