Kaolin was dealuminated until the SiO 2 /Al 2 O 3 molar ratio (R) was raised to values of 2.4 and 2.9mole SiO 2 /mole Al 2 O 3 by chemical reaction with: (i) aqueous HCl; or reaction at high temperature with (ii) NaHSO 4 or (iii) H 2 SO 4 . Dealuminated samples were introduced into a sodium–potassium hydroxide solution under reaction conditions for the gel formation step in zeolite X synthesis using kaolin plus additional Si.Samples dealuminated with HCl showed Si reaction yields (X Si ) in the range of 0.10–0.20, slightly higher than the values obtained with calcined kaolin or calcined kaolin plus quartz. On the other hand, Al reaction yields (X Al ) were in the range of 0.02–0.03, similar to the values obtained for kaolin plus additional Si. Samples dealuminated with NaHSO 4 showed X Si values in the range of 0.50–0.60, above those obtained for kaolin plus additional Si. Conversely, X Al values were in the range of 0.04–0.05, resembling the values obtained for kaolin plus additional Si. In contrast, samples dealuminated with H 2 SO 4 showed X Si values in the range of 0.40–0.50, above the values obtained for kaolin plus additional Si. At the same time, X Al reached values resembling those obtained with samples dealuminated with NaHSO 4 for long reaction times, or kaolin plus additional Si.The values of X Si can be explained in terms of total pore volume and fractal surface dimension deduced through N 2 isotherms at 77K.