The pentafluorohalogenobenzenes react with nucleophiles mainly at the position para to the halogen; ortho replacement occurs to a lesser extent and diminishes in the order C6F5Cl> C6F5Br> C6F5I∼ C6F5H. This is rationalized in terms of an electronic effect, which involves electron repulsion by halogens in π-electron systems and also by steric factors.