Induction times in supersaturated calcium oxalate (CaOx)-the major component of most kidney stones-solutions were determined at 37 degrees C using UV-vis spectrometry with and without Khella extract. The slope of the light absorbance measurement curve indicated the inhibition of calcium oxalate nucleation with Khella extract. The induction time was estimated from the time vs. absorbance curve. Khella seeds were obtained from two sources, one in Turkey and one in Egypt. High performance liquid chromatography (HPLC) results showed that only Turkish Khella extract contained khellin and visnagin that are believed to be the active components of the herb. Both extracts contained calcium, magnesium, and oxalate. It was found that both Khella extracts reduced the induction time at every supersaturation ratio. Using an equation that relates induction times and supersaturation ratios, free energy barrier, and critical nuclei radius were calculated. The results revealed that decrease of free energy barrier and critical nuclei radius as supersaturation ratio increased. In addition, the calculated surface energy of calcium oxalate crystals was decreased from 9.01 to 6.79 and 6.40 mJ/m(2) with Egyptian Khella extract and Turkish Khella extract, respectively. Scanning electron microscopy (SEM) photomicrographs showed that the control supersaturated CaOx solutions produced CaOx monohydrate (COM) crystals. With the addition of Khella extract, the resulting crystals were modified to calcium oxalate dihydrate (COD) form. (C) 2009 Elsevier B.V. All rights reserved.