Over-cracking of Fischer-Tropsch wax remains an obstacle toward successful integration of FT synthesis and upgrading with high middle-distillate yields. The effect of syngas CO on the hydrocracking of n-hexadecane over Pd/H-MFI zeolites with different metal loadings has been investigated. Secondary cracking is miniinized with increasing metal loading in Pd/H-MFI zeolite catalysts, and surprisingly a commensurate n-hexadecane conversion loss is observed, whereas in Pd/H-Y zeolites activity increases with metal loading. By means of tracer chromatography with cyclohexane, and using the solutions of the bidisperse model, it is demonstrated that metal loaded on the H-MFI zeolite lowers the micro- and macropore diffusivities. The presence of CO results in increased activity and concomitant secondary cracking on Pd/H-MFI with high metal loading, reminiscent of a Pd/H-MFI with low metal loading, showing that Pd is only partially poisoned. Pd/H-Y with CO displays lower activity like a Pd/H-Y with less Pd and Pt/H-MFI loses all hydrocracking activity. Severe secondary cracking in the presence of CO attests to an increased distance between metal and acid sites due to Pd migration from the micropores. (c) 2014 Elsevier Inc. All rights reserved.