Experimental work was performed to elucidate the role of acids, including HCl, H2SO4, H3PO4 and CH3COOH, to adjust the pH value of n-BTMS solutions, in the preparation of hydrophobic TiO2. The main findings are as follows : 1. Preparation of hydrophobic TiO2, using an n-BTMS solution consisting of n-BTMS-EtOH-H2O(pH3.3), by the dry coupling treatment method was achieved most quickly with HCl, followed by H2SO4 and CH3COOH. With H3PO4, it was impossible to reach the maximum M value, which is a parameter of hydrophobic degree, under the experimental conditions employed. 2. The resulting time required to achieve the same average molecular weight and average polymerization degree of n-BTMS oligomer of solution system consisting of n-BTMS-EtOH-H2O(pH3.3), was in the sequence of HCl>H2SO4>H3PO4>CH3COOH. 3. No matter which acid was used, an increase of drying temperature resulted in a quick increase of the M value, followed by a stagnation period. 4. When wet treatment methods were used to apply n-BTMS to TiO2, an increasing rate of M value was found in the sequence of HCl>H2SO4>H3PO4>CH3COOH. On the contrary, with a fixed amount of n-BTMS polymer on TiO2, less than 30% of the theoretical amount was needed to produce the maximum M value with CH3COOH. With other acids, approximately twice as much fixed polymer was required. This fact indicates that the fixed polymer amount does not correspond directly to maximum M values.