<p>We studied the suppression of radioactive Cs elution from municipal solid waste incineration (MSWI) fly ash using 10-[(<i>tert</i>-butyldiphenylsilyl)oxy]-decanoic acid. The synthesized decanoic acid derivative coated on MSWI fly ash containing less than 8000&#xa0;Bq&#xa0;kg⁻¹ of radioactive Cs, and the amount of radioactive Cs eluted into pure water was measured by the Japanese Leaching Test No. 13. The elution rate of ¹³⁷Cs from raw fly ash was normalized to 100%, and the relative elution rate of ¹³⁷Cs using 10-[(<i>tert</i>-butyldiphenylsilyl)oxy]-decanoic acid was 25.3%. The synthesized carboxylic acid showed a high elution suppression effect on ¹³⁷Cs in MSWI fly ash. SEM revealed that the water repellent layer was maintained on the MSWI fly ash treated with carboxylic acid synthesized around JLT-13. In addition, the contact angle measurement showed that the surface of the MSWI fly ash treated with the synthesized carboxylic acid for water repellency was hydrophobic. These results indicate that the surface of the&#xa0;MSWI fly ash was changed by applying a water repellent treatment with the synthesized carboxylic acid. </p>