The effect of alkali impregnation on alumina catalysts has been investigated by a physicochemical study of pure and modified alumina catalyst samples. The stereospecificity and selectivity of dehyration reactions, as well as the incidence of ..gamma..-elimination, have been studied by passing suitable substrates over catalyst samples. There was a change in the acidity-basicity balance in the sodium-impregnated alumina samples vis a vis pure alumina, while the surface area virtually remained constant. A higher propensity for ..gamma..-elimination was noticed with increases in basicity of the catalyst. 1-Olefin formation was found to be larger in more basic alumina- and thoria-catalyzed dehydration reactions. Thoria was strikingly unique in its capacity to dehydrate only alcohols, which have at least one ..beta..-hydrogen atom. Neopentyl alcohol could not be dehydrated even under drastic conditions. The modes of elimination in the case of alumina and thoria have been shown to be anti and syn, respectively, from the results of the dehydration studies with threo-3-methyl-2-pentanol. Studies of alcohols with proper ..beta..-substituents revealed that the cis preference is not universal in all catalytic eliminations but, in fact, depends on the mode of elimination. While cis-preference was noticed in alumina-catalyzed anti eliminations, trans-olefin was formed to a major amount inmore » thoria-catalyzed syn-elimination processes. 9 figures, 13 tables.« less