Atom transfer radical polymerization (ATRP) equilibrium constants (K-ATRP) were determined using modified Fischer's equations for the persistent radical effect. The original Fischer's equations could be used only for low conversion of Cut to X-Cull and consequently for relatively low values of K-ATRP. At higher conversion to X-Cu-II (> 10%) and for larger values of K-ATRP (> 10(-7)), modified equations that take into account the changes in catalyst and initiator concentrations should be used. The validity of new equations was confirmed by detailed kinetic simulations. UV-vis spectrometric and GC measurements were used to follow the evolution of X-Cull species and the initiator concentration, respectively, and to successfully determine values of K-ATRP for several catalysts and alkyl halides. The effect of structure on reactivities of ATRP components is presented.