The chemical composition, structural features and surface active properties of alkali lignin, separated from the hydrolyzate of the birch wood sawdust treatment in a moderately alkaline water solution at a temperature below 100 °C, were studied. The separated lignin was rich in p-oxyphenylpropane fragments and carbonyl groups, but contained a relatively low amount of aliphatic hydroxyl and methoxyl groups. The carboxyl groups were weaker than those in kraft lignins. The lignin aqueous solutions were structured, containing both nano- and micro-sized colloidal particles. The pronounced amphiphilic character of the lignin molecules governed their low critical micelle concentration (CMC) values and enhanced the surface activity at the water-air and liquid-liquid interfaces. When decreasing the pH and increasing ionic strength, the surface activity of lignin at the air-water interface grew, while that at the oil-water interface dropped. Scanning electron microscopy (SEM) images revealed that the microstructure of the lignin surface consisted of globules, forming extended clusters of the "grape- bunches" type.