An in-depth study of the structure of castor oil by H-1 and C-13 n.m.r. has allowed the quantitative determination of conversions of alcohol groups in tridimensional polycondensation of castor oil with diisocyanates. Fatty acid methyl ester gas chromatography (FAME-g.c.) and fast bombardment mass spectrometry (FAB-m.s.) examinations of intact glycerides separated by semi-preparative reverse phase high performance liquid chromatography (RP-h.p.l.c.) show that castor oil’s number average hydroxyl functionality ((F) over bar(n)(OH)) of 2.7 (by chemical analysis) results from the contributions of 70% of triol (triricinoleate of glycerol) and 30% of diols (triacylglycerols having only 2 ricinoleyl groups). No monoalcohol (triacylglycerols having 1 ricinoleyl group) was detected. The calculated weight-average hydroxyl functionality ((F) over bar(W) (OH)) and hydroxyl polydispersity index (I(OH)) are 2.8 and 1.03 respectively.