Rh/MOF-5 catalysts have been prepared and characterized by XRD, FTIR, nitrogen adsorption, and TEM. MOFs with different textural properties, namely nano-sized materials and larger crystals, were obtained by changing synthesis conditions. The catalytic properties of the different samples were tested in the hydroformylation reaction of olefins. It is shown that the catalytic properties of nano-sized Rh/MOF-5 catalysts markedly differ from that of larger crystals. Especially, the selectivity to n-aldehydes is clearly improved. However, also the double bond isomerization side reaction is enhanced. The results confirm the potential of MOFs for the application as catalyst supports.