The hydrophile-lipophile balance of nonionic surfactants changes from hydrophilic to lipophilic with the rise in temperature and is considered to be just balanced in a temperature range at which a lamellar liquid crystalline phase forms in the surfactant-water system. The mid-temperature of that range is called the liquid crystal temperature, T LC. It was found that the following relation between the HLB numbers of nonionic surfactants and the LC temperature in an aqueous system is valid: T LC = k H 2 O (∑N HLB (i)W (i) − N H 2 O) where k H 2O = 17°C/HLB unit, N H 2O = 7.4 HLB unit, N HLB ( i) and W ( i) are the HLB number and the weight fraction of ith surfactant in total surfactants. This relationship is similar to that between the HLB number and the HLB temperature in a water/nonionic surfactant/oil system. Since the surfactant ratio to produce a lamellar liquid crystalline phase in a diluted region is almost independent of temperature in an ionicnonionic surfactant system, the condition to produce a lamellar liquid crystal was found at 30°C to be ∑N HLB (i)W (i) = 9.2 . This relation is equivalent to the above equation and is valid for an ionic-nonionic surfactant aqueous system.