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Critical concentrations of lyotropic liquid crystalline ethylcellulose in more than tensolvents were determined using both Abbe refractometer and polarized microscopy. Criti-cal concentration C

crit

of forming liquid crystal phase decreased with increasing solubilityparameter δ of solvent until approaching the δ of polymer. Although the alcohols usedas solvents had the same variation rule

the critical concentration values of their solutionswere much higher

due to their excessive large hydrogen bond component of δ. The experi-ments of using mixed solvents which showed good linear relation between C

crit

and δ alsoproved this rule. A technique of Transmission Optical Analysis was first used to estimatethe concentration dependence of critical phase transition temperature T

crit

of EC

and aT-C phase diagram could be drawn.