The solubilities of heterogeneous atom transfer radical polymerization (ATRP) catalysts are determined for the first time and used to calculate previously unquantified ATRP equilibrium constants (K-ATRP). These new data are essential for understanding, modeling, and designing ATRP processes with low catalyst concentrations. K-ATRP values for (CuBr)-Br-I/1,1,4,7,10,10-hexamethyltriethylenetetramine ((CuBr)-Br-I/HMTETA) and (CuBr)-Br-I/N,N,N',N',N ''-pentamethyldiethylenetriamine ((CuBr)-Br-I/PMDETA) are 8.66 x 10(-6) and 1.44 x 10(-6), respectively. The limited solubility of the (CuBr)-Br-I/HMTETA catalyst explains why large reductions in metal salt concentration can be made without affecting polymerization rates. Catalyst solubility and polymerization rate increase with ligand concentration. In contrast, increasing the metal salt concentration in excess of the ligand's causes a drop in catalyst solubility. This unexpected observation is attributed to the formation of insoluble catalyst networks (gels). The solubility data point to differences in the ionic character of the catalyst complexes formed. The following solubility trend is observed at ATRP conditions (toluene, 90 degrees C): (CuBr)-Br-I/PMDETA >> (CuBr2)-Br-II/PMDETA > (CuBr2)-Br-II/HMTETA > (CuBr)-Br-I/HMTETA.