The well-known laxative effect of coffee is not actually due to caffeine. It is the acidity, from the chlorogenic acid present in both regular and decaf coffee that acts on the gastric acids in the stomach, helping to stimulate the colon to eliminate waste from the body. So decaf coffee does act as a laxative, but studies have shown that the effect diminishes over time, as the body gets used to it and builds up more tolerance to the acidity levels.