The effect of temperature on the molecular weight distribution of kraft lignin during its dissolution in Pyridinium Acetate PyAce a protic ionic liquid was examined in detail by gel permeation chromatography. Lignin solubility increased with increasing temperature from 50°C to 100°C. The average molecular weight (Mw) of PyAce treated and recovered kraft lignin after thermal treatment with PyAce decreased from 4119 g mol-1 to 1995 g mol-1 within the temperature range of 50°C-75°C. A significant increase in molecular weight for PyAce treated lignin was observed as the process temperature was further increased from 75°C to 100°C. Results indicated that the condensation reactions occurred at elevated temperatures resulting in increased molecular weight during the dissolution process. © 2016 The Authors.