Raman microscopy has been applied to study the preparation of shaped Mo/Al2O3 catalysts. The speciation of different Mo complexes over gamma-Al2O3 support bodies was followed in time after pore volume impregnation with aqueous solutions containing different Mo complexes. The addition of NO3- to the impregnation solutions allows for a quantitative Raman analysis of the distribution of different complexes over the catalyst bodies as this ion can be used as an internal standard. After impregnation with an acidic ammonium heptamolybdate (AHM) solution, the strong interaction between Mo7O246- and Al2O3 results in slow transport of this complex through the support and extensive formation of Al(OH)(6)Mo6O183- near the outer surface of the support bodies. This may be prevented by decreasing the interaction between Mo and Al2O3. In this way, transport is facilitated and a homogeneous distribution of Mo is obtained on a reasonable time scale. A decrease in interaction between Mo and Al2O3 can be achieved by using alkaline impregnation solutions or by the addition of complexing agents, such as citrate and phosphate, to the impregnation solution. In general, time-resolved in situ Raman microscopy can be a valuable tool to study the physicochemical processes during the preparation of supported catalysts.