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The effect of adding SiO$\_2$ 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 Mössbauer spectroscopy, BET surface area, XRD and SEM characterization methods. Adding SiO$\_2$ to the catalyst during precipitation or immediately after precipitation (i.e., precipitated SiO$\_2$) 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 SiO$\_2$ matrix. Consequently, these crystallites are resistant to reduction and carburisation. When SiO$\_2$ was added to the catalyst after heat treatment (i.e., binder SiO$\_2$), the resulting catalyst was observed to consist of segregated SiO$\_2$-rich and Fe-rich phases. The distribution of K$\_2$O in both these phases indicates that the amount of effective K$\_2$O, i.e., that associated with Fe, is less when SiO$\_2$ is added as a binder. The low extent of reduction and carburisation observed with catalysts that contain precipitated SiO$\_2$ 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.