We have quantum chemically explored arylic carbon-substituent bond activation via oxidative insertion of a palladium catalyst in C6H5X + PdLn model systems (X = H, Cl, CH3; Ln = no ligand, PH3, (PH3)2, PH2C2H4PH2) using relativistic density functional theory at ZORA-BLYP/TZ2P. Besides exploring reactivity trends and comparing them to aliphatic C-X activation, we aim at uncovering the physical factors behind the activity and selectivity. Our results show that barriers for arylic C-X activation are lower than those for the corresponding aliphatic C-X bonds. However, trends along bonds or upon variation of ligands are similar. Thus, bond activation barriers increase along C-Cl < C-H < C-C and along Pd < Pd(PH3) or Pd(PH2C2H4PH2) < Pd(PH3)2. Activation strain analyses in conjunction with quantitative molecular orbital theory trace these trends to the rigidity and bonding capability of the various C-X bonds, model catalysts, and ligands.