A major difficulty in processing thermoplastic composites is to achieve cost-effective and high-quality impregnation of the fibre reinforcement by the matrix resin. The aim of this work is to improve impregnation prior to in-situ polymerisation of Polyamide 12 (PA12) by exploiting a very low monomer viscosity (7.10-3 Pa.s). A Time, Temperature and Transformation (TTT) diagram has been established to map the different events occurring during the PA12 polymerisation. Polymerisation temperatures above as well as below the final polymer melting point (Tm8) have been investigated. It was shown that two events occur during polymerisation below Tm8: monomer-polymer phase separation and polymer crystallisation. The polymerisation kinetics of anionic PA12 were studied and the viscosity change during polymerisation was measured to determine how the processing temperature influences the impregnation time. These results define a processing window for composite materials based on in-situ polymerisation of PA12.