Abstract: Water stop compositions and methods of preparing same are disclosed. The compositions exhibit a controlled swelling or volumetric expansion upon immersion in water. The compositions swell about 5-8% after 1 day's immersion and about 35-65% after 28 day's immersion. A blend of fast acting sodium bentonite clay and a slower acting calcium bentonite clay contribute to this desirable controlled expansion along with a specific combination of thermoplastic elastomer, cross-linked butyl rubber components and polyisobutylene. Another important factor in providing the desired expansion is a thorough mixing step to ensure product homogeneity whereby mix components are blended in a Sigma mixer for an excess period of time after all components have been added to the mixer.An exemplary water stop composition includes about 10-30 wt % elastomer, about 15-30 wt % filler, about 20-40 wt % plasticizer, and about 25-35 wt % water swellable clay, the foregoing percentages adding up to 100 wt %.