Titanium silicate-1 (TS-1) samples contained a significant amount of potassium derived from contaminants in an aqueous solution of Pr4NOH. By washing with H2SO4, the alkali-exchanged sites were converted to sites with two OH groups or 'closed' Ti(OSi)(4) sites. Treatment of the acid-washed samples with an aqueous solution of K2CO3 led to the incorporation of potassium with a K/Ti ratio of around unity, suggesting that K ion exchange on TS-1 corresponds to the presence of Ti in the framework. The potassium ion exchange is considered to occur mainly on the Si-OH group adjacent to the 'open' Ti site. The presence of potassium retarded the oxidation of organic compounds with aqueous H2O2 to a different extent depending on the substrates. The retarding effect decreased in the order hexane>2-hexene>2-penten-1-ol and there was little inhibition in the oxidation of 2-penten-1-ol. This could be interpreted in terms of polarities of substrates and active sites in TS-I; non-polar hexane was less easily accessible to the Ti environment made more polar by adjacent potassium. In a similar manner, promotion of the oxidation of the alcoholic group in the reaction of 2-penten-1-ol over the K-type TS-I might be ascribed to increased interaction of the alcoholic group with the active Ti site.