The Methanol-to-Olefins (MTO) process for the production of polymer-grade olefins is a possible stepin the upgrading of natural gas. The preferred MTO catalyst is the microporous silicoaluminophosphateH-SAPO-34 (CHA topology). In the present work, H-SAPO-34 is compared with H-SSZ-13which has the same topology and density of acid sites, but is slightly more acidic due to the frameworkcomposition. This is a one-parameter study where the effect of acidity on the MTO-process isinvestigated. Both materials are very similar as catalysts for the conversion of methanol, the effluentand retained hydrocarbons contain mainly the same compounds. It is found that H-SSZ-13 has alower total conversion capacity than H-SAPO-34. Both catalysts displayed an optimum temperature,but under the same conditions (Weight Hourly Space Velocity=6.2 hr-1) the optimum temperatureof H-SSZ-13 (350° C) was 50°C below that of H-SAPO-34 (400° C). The production of retainedhydrocarbons was faster in H-SSZ-13, which also produced larger ring compounds than H-SAPO-34at the same conditions. Upon flushing with He it was possible to discriminate between the active andnon-active compounds in the retained material. The more methylated benzenes were the most reactiveones in both catalysts. In H-SAPO-34 at 350° C only penta- and hexamethylbenzene were active,while also tetramethylbenzene was active in H-SSZ-13. When decreasing the temperature to 300°C,tetramethylbenzene was no longer protonated in H-SSZ-13, so it appears that higher acid strengthhas an effect comparable to increasing the temperature. Changing WHSV had a great impact on thecatalyst properties, such as optimum temperature and conversion capacity. Halving WHSV increasedthe optimum temperature of H-SAPO-34 to 450° C.A theoretical study of the adsorption of methanol and CO on the acid sites of the two catalysts isalso presented in this thesis. Frequency calculations for the adsorption of CO showed that H-SSZ-13was slightly more acidic than H-SAPO-34, in good agreement with experimental results reported inthe literature. The calculated methanol adsorption energies were higher than reported in previousstudies, Furthermore, the calculations indicated that methanol tends to point into the cage rather thanthe 8-rings of the structure. This is not in agreement with studies reported earlier.