To understand why the Siralox materials show different activities in the aldol condensation of acetone, their nature and number of acid sites are assessed. The number of acid sites initially increases by doping γ-Al2O3 with 1 wt% silica from 0.21 to 0.49 mmol g -1 . However, the number of acid sites starts decreasing from 20 wt% SiO2 and is strongly decreased for high silica-loadings as in Siralox 70 with 0.065 mmol g -1 (Figure ). No direct correlation with mesitylene yield can be found for the total number of acid sites obtained by NH3-TPD, indicating the importance of specific active acid sites. The desorption temperature of the chemisorbed NH3 is an indicator for the strength of the probed acid sites, but in the case of the ASA, the large variety of acid sites leads to a broad desorption curve which renders it difficult to quantify or select individual sites for comparison based on their acid strength. Instead, the temperature of the desorption maximum is used for estimation as it indicates the bond strength of a majority of acid sites. Analogous to the number of acid sites, low silica-doping leads to an increase in the maximum desorption temperature from 260 °C to approximately 270 °C. For higher silica-contents, except for Siralox 40, the maximum temperature declines. Considering that the highest yields of mesitylene were found for Siralox 20-40, a slightly decreased number of acid sites and strength appears beneficial for the stable formation of mesitylene found for ASA. This could be attributed to the fact that a high density of strong acid sites leads to faster deactivation due to the favored formation of larger byproducts, as seen for zeolite HY. By comparing the dimer yield with the NH3-desorption temperature, it becomes apparent that mesityl oxide benefits from the higher density of slightly stronger acid sites. As for the dimerization two acetone molecules need to react (and possibly be activated in close vicinity), the yield of mesityl oxide is mainly dictated by the amount and strength of acid sites. Due to the structural complexity of ASA with multiple types of acid sites, results obtained by NH3-TPD are insufficient to explain the observed trends. Therefore, the nature and number of acid sites on which the reactions take place must be considered in more detail.