Methyl oleate and methyl elaidate (two industrially relevant FAMEs, methyl esters of fatty acids) were hydrogenated in a batch slurry autoclave using a supported palladium catalyst. The mixture of reaction products formed as a result of the competitive reaction network of positional isomerization, geometrical isomerization, and hydrogenation were analyzed using gas chromatography and combined peak fitting. Diffusion limitations, even at low Thiele modulus, were found to have a significant impact on the product distribution. Depending on the starting material, either the cis or the trans double bonds apparently migrated faster along the carbon chain. Also, with increasing Thiele modulus, the conversion level at which cis-transequilibrium was reached increased: if significant intraparticle gradients were present, the equilibrium was never reached. Several of these phenomena have previously been ascribed to different kinetics of the various isomers, which would stem from the differences in adsorption strength. For the experimental data of this work, mass transfer effects were found to satisfactorily account for the differences in product composition using a simple reaction-diffusion model for all the isomers involved, without needing to assume significant differences in adsorption strength. The result presented in this work gives fundamental insight into the product composition of fat hardening processes.