The phthiocerols, RCH(OMe)·CHMe·[CH2]4·CH(OH)·CH2·CH(OH)·[CH2]nMe (R = Et or Me, n= 20 and 22), constituents of tuberculolipids, have been converted, by substituting H for each OH, into the corresponding methoxyphthioceranes A and B (R = Et or Me, respectively). An optically active diastereoisomer of the latter and structurally related model compounds have been synthesized. By comparison of the methoxyphthioceranes with the synthetic products the asymmetric centres bearing the methoxy-group and the methyl branch are assigned the threo-configuration, threo being used in the sense defined in J.C.S. Perkin I, 1973, 109.