Four lignin samples (Alcell, Indulin AT, Curan 27-11P, and Sarkanda) were converted into liquid polyols by a chain extension reaction with propylene oxide (oxypropylation). The reaction was carried out in a batch reactor using potassium hydroxide (KOH) as the catalyst. The formulation was optimized with Alcell lignin by modifying the lignin/propylene oxide ratio [L/PO (w/v)] and catalyst content [C/(C + L) (% w/w)]. L/PO ratios of 10/90, 20/80, 30/70, and 40/60 and catalyst contents from 2% to 5% were used. The influence of the studied variables was examined by following the evolution of the hydroxyl index, viscosity, homopolymer content, and molecular weight distribution. The optimal oxypropylation conditions were established according to the requirements that a given polyol should fill when used in rigid polyurethane foam formulations, i.e., a hydroxyl index between 300 and 800 and a viscosity below 300 Pa.s. The established optimal conditions were applied to three formulations based on the other lignin samples, namely, 20/80/5, 30/70/2, and 35/65/2. The produced polyols were characterized, and the determined values for hydroxyl index and viscosity were found to be in the range of those typical of commercial polyols employed for polyurethane rigid foams.