Changes in the quantitative composition of triacylglycerols (TAGs) in maturing sea buckthorn (Hippophaë rhamnoides L.) seeds were determined by lipase hydrolysis. As a whole, the rate of synthesis of separate TAG classes increased in proportion to both their unsaturation and relative content (weight percent) in total TAGs. Up to the 80th day of maturation, the formation of triunsaturated TAGs was predominant. Subsequently, at the terminal stage of seed ripening, the absolute content (in nanomoles per seed) of a major group of these TAGs containing linolenic and linoleic acyls decreased by ∼7%, and the increase in the total TAG content was mainly due to the synthesis of TAG molecules including stearic and palmitic acyls in the rac-1,3 positions, as well as those containing oleate in the sn-2 position. At each maturation stage, the composition of the TAGs formed was controlled both by the composition of fatty acids available for TAG synthesis and by the rate of incorporation of a particular fatty acid into the sn-2 position of the TAGs. Keywords: Sea buckthorn; Hippophaë rhamnoides L.; seed maturation; fatty acids; triacylglycerols; lipase hydrolysis; sn-2 acylation of glycerol; positional-type composition; positional-species composition; kinetic constant of relative rate of biosynthesis