ZSM-5 and Silicalite-I were synthesized hydrothermally using Zeosil 77 and sodium silicate solution as silica sources. The variations of pH in the aging step at room temperature and during the hydrothermal reaction period, and the effect of modification of pH with sulfuric acid were investigated in relation to the nucleation process of the zeo-lites. Without aging treatment, pH of the substrate was decreased significantly during the induction period, but it appro-ached to a constant value asymptotically after the induction period. The same phenomena have been observed in the aging step, which seem to be a consequence of the nuclei being formed by the reaction of OH-with silica particles pro-ducing silanol or soluble silicates ions. For the synthesis of ZSM-5 from the substrate composition of 11.7 Na₂O-Al₂O₃-90 SiO₂-3510 H₂-O-10.8 TPABr, by adjusting pH of the substrate with sulfuric acid and without aging, the best result was obtained from the substrate which the pH was adjusted to 11.5-11.0 from 13.2 of originals. Even from a substrate which has unsuitable composition for ZSM-5 synthesis due to high Na⁺ content and pH, i.e. 30.6 Na₂O-Al₂O₃-90 SiO₂-5049 H₂O-9.0 TPAOH, ZSM-5 in high purity could be obtained by adjusting the pH of the substrate to 11.0 from 13.7 with H₂SO₄. The faster crystallization rate was resulted in the synthesis of Silicalite-I than in ZSM-5, and the rate of Silicalite-I crystallization, in contrast to ZSM-5, was inhibited by controlling pH of the substrate from about 13 of original to 11.0. Formation of Silicalite-I, A Al-free Pentasil zeolite, prefers a bit higher pH of the substrate than for ZSM-5. As the template, the rapider reaction rates were achieved with TPABr than with TPAOH, probably owing to the pre-ferable pH of the substrate by using TPABr.