Epidemiological data suggest that increased risk of colon cancer is correlated with a higher fecal pH. Although some experimental studies have shown a protective effect against experimentally induced colon cancer by acidifying colonic contents, others have shown that a more acidified colonic content is associated with increased cell proliferation and enhanced tumorigenesis. It is now clear that simply acidifying colonic contents will not consistently result in decreased tumorigenesis. Perhaps the key is how colonic contents are acidified--a decrease in base production or an increase in acid production. Or, more important than luminal pH itself, may be a factor affected by changes in hydrogen ion concentration. This paper reviews the determinants of colonic luminal pH and their dietary sources and discusses important physiological consequences of modifying the pH of colonic contents.