Chromatographic, chemical, and spectroscopic techniques were used to characterize the physicochemical properties of napin purified by preparative chromatography. The molar extinction coefficient was determined (ε=0.56), and static and dynamic light scattering measurements enabled the average molecular weight (Mw ) 13919), the second virial coefficient (A2 ) 23.95 χ 10-5 mol cm3 g-2), and the hydrodynamic radius (RH ) 1.98 nm) to be determined. No conformational changes were observed by fluorescence and circular dichroism measurements in different buffers at pH 3, 4.6, 7, and 12, confirming the high pH stability of this protein. From MALDI-TOF analysis and after enzymatic digestion, it was found that this purified sample, extracted from the rapeseed variety Express, contained mainly isoform 2SS3_BRANA.