This paper addresses the question of why some volcanic ashes are zeolitized and others are not. The pH of interstitial water is the most important control on zeolitization. Surface water and ground-water in volcanic areas is acidic, but zeolitization requires neutral to alkaline conditions. Therefore, acidic interstitial water must be removed or neutralized before zeolitization can take place. Conditions favoring zeolitization are deposition or redeposition under water, or reheating of ash after acidic interstitial water has been removed, or influx of slightly alkaline (bicarbonate-bearing) groundwater from overlying or neighboring limestones.