The effect of the introduction of potassium into namoscale HZSM-5 upon acidity and catalytic performance in n-octene reaction was investigated. A modified method of Hammett indicator was used to quantitatively determine the acid strength distribution of the catalysts. The acid sites with acid strength Ho <= -3.0 (0.20 mmol/g), -3.0 < H(0) <= +2.27 (0.10 mmol/g) and +2.27 < H(0) <= + 4.8 (0.80 mmol/g) were detected over the nanoscale HZSM-5. The strong acid sites with acid strength H(0) <= +2.27 were indispensable to the transformation of n-octene into aromatics. Isomerization and cracking of n-octene were apt to take place on the weak acid sites with acid strength +2.27 < H(0) <= + 4.8. With decreasing the amount of the strong acid sites, the selectivities to benzene and toluene decreased, while that to C(9+) aromatics increased. (C) 2008 Elsevier Ltd. All rights reserved.