By use of cytochrome P-450 as the prototype, kinetic descriptions are derived for the observed isotope effects for several models of enzymatic systems which are capable of generating multiple products from single substrates. The models include rapid and slow equilibria between enzyme-substrate orientations as well as multiple simultaneous and multiple sequential isotope effects. When an equilibrium is established between enzyme-substrate complexes that are responsible for the oxidation of different positions of the substrate, the kinetics can be represented by competing pathways from the same intermediate. When direct interchange between the complexes does not occur, the alternate pathway mimics the presence of a competitive inhibitor in the substrate solution. In general, the presence of alternate pathways in competition with the isotopically sensitive step will tend to unmask the intrinsic isotope effect.