A novel liquid system has been developed to initiate the anionic polymerization of lactam-12. This system, containing both an activator and a catalyst, has the primary advantage over previous systems of permitting infinite storage of the reactant, and it avoids premixing of batches. The anionic species of the system were identified with matrix-assisted laser desorption/ionization time-of-flight measurements, and Fourier transform infrared was used to measure the change in their concentrations during the polymerization. A guanidine anion was formed, and the system initiated the polymerization by this guanidine and by the catalyst. The kinetics of the anionic polymerization of lactam-12 into polyamide-12 were followed with differential scanning calorimetry measurements. The determined reaction rates indicated that this liquid system was particularly well suited for initiating in situ polymerization during liquid molding.