The stericcourse of some reactions of 2,3-dihalotetrahydropyrans with sodium methoxide and with methyl- and phenyl-magnesium bromides has been reinvestigated. Whereas the methanolysis of the - and -dichlorides occurs with practically complete inversion, it has been confirmed that their reactions, as well as those of the dibromides, with methyl-magnesium bromide are non-stereospecific, yielding mixtures