Atom transfer radical polymerization (ATRP) equilibrium constants (KATRP) were measured during polymerization of methyl acrylate (MA) with CuIBr/CuIIBr2 in either dimethyl sulfoxide (DMSO) or acetonitrile (MeCN) in the presence of either tris(2-pyridylmethyl)amine (TPMA) or tris[2-(dimethylamino)ethyl]amine (Me6TREN) as the ligand and with ethyl 2-bromopropionate as the initiator. The ln(KATRP) values changed linearly with the volume fraction of solvents in the reaction medium, allowing extrapolation of the values for KATRP to bulk conditions, which were 2 × 10-9 and 3 × 10-8 for TPMA and Me6TREN ligands at 25 °C, respectively. The temperature effect on KATRP values was studied in MA/MeCN = 1/1 (v/v) with TPMA as the ligand in the temperature range from 0 to 60 °C. The KATRP values increased with temperature providing ΔH = 36 kJ mol-1 in MeCN.