The performance of light naphtha isomerization catalysts is greatly influenced by catalyst preparation and pretreatment, and optimal catalyst pretreatment conditions vary depending on feed properties. Experimental results demonstrated that catalyst performance was determined by Pt dispersion when using pure feed, whereas it was determined by both Pt dispersion and Pt cluster stability for feed containing 500 ppm sulfur. Tested with pure feed nearly all catalyst samples fit a pattern which the isomerization selectivity increased with greater Pt dispersion, The only two exceptions, samples which gave unusually low conversion, have been due to pore mouth plugging which we infer from FFT-IR and XRD results. Highest Pt dispersion and, thus, best performance with pure feed was obtained with a catalyst prepared by the ion exchange and pretreated at a low temperature-ramping rate (0.5 degrees C/min) during calcination and reduction. In addition, this catalyst was pretreated by calcination at 450 degrees C, followed with reduction at 450 degrees C. In contrast, the catalyst having the best performance with feed containing 500 ppm sulfur was pretreated by calcination at 450 degrees C, followed with reduction at 530 degrees C; the superior performance may result from a compromise between metal dispersion and metal cluster stability.