The conversion of waste incineration fly ash to zeolitic compounds by hydrothermal treatment in the presence of NaOH solution has been investigated. An acid pretreatment of fly ash in addition to water washing and heating at 800degreesC was found to be an important process to prepare reproducible zeolitic compounds. Treatment with a low-concentration NaOH solution at 60degreesC and 100degrees-120degreesC successfully produced zeolites A and P, respectively, as major products. At increased concentrations of NaOH, sodalite was generated. The hydrothermal products showed uptake behavior for NH4+ ions.