The fibrillation tendencies of various cellulosic fibers in aqueous solution containing alkali metal hydroxide and ethanol were evaluated with two specific parameters: The critical point of fibrillation (CPFconc.), that is a concentration of swelling agent where the fibrillation begins, and the ratio of initial increase in fibril number to increase in concentration of swelling agent (Ii). The CPFconc. and the Ii are defined as fibrillation stability and fibrillation sensitivity to swelling agent, respectively. Lyocell fiber (CLY1) has the smallest CPFconc. and the largest Ii, representing the lowest fibrillation stability and the highest fibrillation sensitivity, leading to the highest fibrillation tendency in CLY1 among the fibers tested. Although crosslinking improved fibrillation stability in lyocell as compared to modal, the fibrillation stability remained higher owing to the high water capacity and the high affinity for alkali. In alkali solution at the same concentration CLY1 fibrillation increased in the order of LiOH > NaOH > KOH. However, the plot of fibril number against solvent retention value of CLY1 in different alkaline solutions gives a slope of 110 count ·g/cm3 regardless of alkali type, the critical degree of swelling for CLY1 with no fibrillation was 0.62 cm3/g in alkali solutions and 0.45 cm3/g in ethanol/water mixture. © Springer 2005.