Natural clinoptilolite zeolite-CLIN was acidified by ammonium ion exchange followed by calcination and HCl treatment under soft conditions. The influence of the modification type and treatment conditions on the structure, morphology and acidity was studied by XRD, SEM, FTIR, ICP-AES, TG/DSC, ammonia-TPD, 1H, 27Al and 29SI MAS NMR and N2-ads-des. High temperature calcination of NH4-CLIN and concentrated acid has a more severe impact on the clinoptilolite structure. H-CLIN catalysts are potential and active acid catalysts and synthesis-structure-porosity-acidity-activity relationship was found.<eng>