(12-Ricinoleoylricinoleoyl)diricinoleoylglycerol (RRRR), a tetraacylglycerol, was identified earlier in castor oil. Using ESI-MS (4), 95% of the 12-ricinoleoylricinoleoyl chain was identified at the sn-2 position of the glycerol backbone of RRRR. Regiospecific location of the 12-ricinoleoylricinoleoyl chain of RRRR on the glycerol backbone was identified and quantified by the ions from the losses of the acyl chains at the sn-2 position as alpha,beta-unsaturated fatty acids from the lithium adduct of RRRR. The regiospecific location was confirmed by hydrolysis of RRRR using sn-1,3 specific lipase. By comparison to the mass spectrum of 1- O-palmityl-2,3-palmitoyl- rac-glycerol containing one ether bond, the 12-ricinoleoylricinoleoyl chain of RRRR is indeed the ester bond between the two ricinoleoyl chains, not the ether bond formed from the two hydroxyl groups of the two ricinoleoyl chains. The structure of RRRR is 2-(12-ricinoleoylricinoleoyl)-1,3-diricinoleoyl- sn-glycerol.