The activated anionic ring-opening polymerization of epsilon-caprolactam initiated by 0.35 mol% of combined initiator, i.e., equimolar mixture of magnesium di(epsilon-caprolactamate) (CL2Mg) with magnesium halides (MgCl2, MgBr2, and MgI2) as well as of epsilon-caprolactam magnesium bromide (CLMgBr) in the presence of 0.35 mol% of N-acetyl-epsilon-caprolactam as an activator has been investigated in the temperature range 140-200 A degrees C. It was found that the reaction rate increased while the apparent activation energy decreased in the following series: CL2Mg/MgCl2 < CL2Mg/MgBr2 similar to CLMgBr < CL2Mg/MgI2. In addition, the poly(epsilon-caprolactam)s prepared with CL2Mg/MgX2 (MgX2 = MgCl2, MgBr2, and MgI2) are characterized by slightly higher thermal stability than polymers obtained with CLMgBr as initiator. These observations were explained in terms of the coordination of Lewis acids (MgX2, where X = Cl, Br, and I) with imide carbonyl of N-acyllactam end groups leading to the increase of their reactivity and stability.