The effect of adding SiO2 to a precipitated Fe-based Fischer-Tropsch catalyst was investigated. Silica was added to the catalyst either during or after precipitation. The iron-based Fischer-Tropsch catalysts were studied using Mossbauer spectroscopy, BET surface area, XRD and SEM characterization methods. Adding SiO2 to the catalyst during precipitation or immediately after precipitation (i.e., precipitated SiO2) results in the formation of Fe crystallites with an average diameter less than 3 nm, which have high surface areas and exhibit a strong interaction with the SiO2 matrix. Consequently, these crystallites are resistant to reduction and carburisation. When SiO2 was added to the catalyst after heat treatment (i.e., binder SiO2), the resulting catalyst was observed to consist of segregated SiO2-rich and Fe-rich phases. The distribution of K2O in both these phases indicates that the amount of effective K2O, i.e., that associated with Fe, is less when SiO2 is added as a binder. The low extent of reduction and carburisation observed with catalysts that contain precipitated SiO2 results in catalysts with low % CO conversion. A positive correlation between the amount of iron carbides present in the catalyst and the % CO conversion was observed in these studies.