Wide-line proton NMR has revealed differences in the water environment in two zeolitic isomorphs : clinoptilolite and heulandite. The proton spectrum of clinoptilolite is Gaussian in shape, while that of heulandite is a (reduced splitting) Fake doublet. Studies of clinoptilolite from five different locations and heulandite from nine different sites agree with this observation. These results suggest a simple nondestructive NMR method for distinguishing heulandite from clinoptilolite. Dehydration experiments are particularly informative. Proton line widths of clinoptilolite as a function of the dehydration temperature reveal three types of water binding, none being absorbed water. Similar studies with heulandite reveal a change from the Fake doublet to a Gaussian, which is probably associated with the heulandite to heulandite B transformation; Al-27 and Si-29 NMR studies of clinoptilolite indicate a change in framework structure and/or cation binding with dehydration. Al-27 NMR of heulandite exhibits an increase in line width with dehydration temperature to 175 degrees C. At this temperature the increase stops and the line width remains constant to 215 degrees C, the maximum temperature studied. This agrees with the proton studies and is attributed to the heulandite to heulandite B transformation. Si-29 NMR of heulandite reveals a framework structural change and/or cation binding with dehydration. All of these observations are reversible upon rehydration.