A polar cytotoxic fraction of the marine sponge Theonella swinhoei was analyzed to be a mixture of four isomeric compounds, 21,22-dihydroxyonnamides A1, A2, A3, and A4. They were separated after conversion to the isopropyridene derivatives. The structures of the 21S,22S- and 21R,22R-isomers were determined by comparison of their spectral data with those prepared from onnamide A by asymmetric dihydroxylation. During the analysis of NMR data of these derivatives, an empirical rule was implied to assign the relative stereochemistry of linear 1,5-diol. This rule was applied to assign the stereochemistry of the remaining congeners.