The acidity of Beta zeolites with Si/Al ratios ranging from 18 to 33 was determined using appropriate temperature programmed desorption of ammonia conditions and their catalytic behavior was evaluated by disproportionation of ethylbenzene. Samples with greater Si/Al ratios presented higher acid strength by retaining ammonia at higher temperatures and showed a higher ethylbenzene disproportionation activity per aluminum atom. These results indicate that as the aluminum content in zeolite Beta is reduced in that range, the strength of the associated acid sites increases.