A series of 8a-methyldecahydronaphthalen-4a-ols labelled at C(1) with deuterium have been prepared and their reactions in H₂SO₄-Ac₂0-AcOH and SOCl₂-pyridine studied. The dehydration of 8a-methyl-cis-decahydronaphthalen-r-4a-ol-t-5-d (27b) and 8a-methyl-trans-decahydronaphthalen-r-4a-ol-t-5-d (28b) in H₂SO₄-Ac₂O-AcOH occurs with no loss of deuterium. In each case a proton syn to the departing C(9)- moiety is lost. No products of skeletal rearrangement or methyl migration were detected. In the dehydration of 8a-methyl-trans-decahydronaphthalen-r-4a-ol-c-5-d (28g) in H₂SO₄-A₂O-AcOH significant loss of the C(1) deuterium occurs. Reaction of 5a-cholestan- 5-ol-4a-d (3f) with H₂SO₄-Ac₂O-AcOH gives a mixture (1:1) of cholest-4-ene (14a) and cholest-5-ene-4a-d (7d). The results of these studies have been rationalized in terms of the intermediacy of a tight ion pair in which the departing oxy-anion acts as base in the abstraction of an adjacent proton. The results of dehydration of 5a-methyl-cis-decahydronaphthalen-r-4a-ol-t-5-d (27b) and 8a-methyltrans- decahydronaphthalen-r-4a-ol-t-5-d (28b) in SOCl₂-pyridine contrast with those obtained for H₂SO₄-Ac₂O-AcOH. Dehydration occurs with 43 and 35% deuterium loss respectively, and the reactions are thought to occur via an E2 type process. Reaction of t-8a-hydroxy-4a-methyl-trans-decahydronaphth-r-l-yl acetate (28h) with SOCl₂-pyridine gave c-4a-methyl-1,2,3,4,4a,5,6,7-octahydronaphth-r-l-yl acetate (23f). Reaction of t-8a-hydroxy-4a-methyl-trans-decahydronaphth-r-l-yl acetate (28h) in H₂SO₄-Ac₂O-AcOH gave c-8a-methyl-1,2,3,4,6,7,8,8a-octahydronaphth-r-l-yl acetate (24b,50%), c-8a-methyl-1,2,3,5,6,7,8,8a-octahydronaphth-r-l-yl acetate (50a,33%), 4a-methyl- 2,3,4,4a,5,6,7,8-octahydronaphth-l-yl acetate (23e, 8%), and 4a-methyl-trans-decahydronaphthalen-l-one (32a,8%). The rearrangement of 4a,5-epoxy-c-8a-methyltrans- decahydronaphth-r-l-yl acetate (25b) with BF₃-etherate in benzene gave c-8a-methyl-1,2,3,7,8,8ahexahydronaphth-r-l-yl acetate (62a,30%), c-8a-methyl-5-oxo-trans-decahydronaphth-r-l-yl acetate (32b,26%), 4a-fluoro-c-5-hydroxy-8a-methyl-trans-decahydronaphthr-l-yl acetate (60c,40%) and an unidentified aldehyde (4%). Rearrangement of 4a,5-epoxy-c-8a-methyl-transdecahydronaphth-r-l-yl acetate (26c) with BF₃-etherate in benzene gave c-8a-methyl-1,2,3,7,8,8a-hexahydronaphthr-l-yl acetate (62a,34%), the aldehyde (65,6%), c-8a-methyl-5-oxo-cis-decahydronaphth-r-l-yl acetate (31b,7%), c-8a-methyl-5-oxo-trans-decahydronaphth-r-l-yl acetate (32b,4%), 4a-fluoro-t-5-hydroxy-c-8a-methyl-cis- decahydronaphth-r-l-yl acetate (61b,22%) and two unidentified compounds (27%). Thermodynamic parameters for the equilibration of the conformational enantiomorphs (37) and (38) of 8a-methyl-cis-decahydronaphthalen-4a-ol have been determined as Ea = 69.8 ± 0.9 kJmol-⁻¹,ΔH‡ = 66.7±1.3 kJmol-⁻¹and ΔS‡ = + 12 ± 5 JK-⁻¹mol-⁻¹. It has been shown that conformer (39) of 4a-methyl-cis-decahydronaphthalen-l-one is 1.25 kJ-⁻¹mol-⁻¹ more stable at 200K than conformer (40) , and the thermodynamic parameters for the conformational interconversion of (39) ’ (40) have been determined as Ea = 42.9 ± 0.7 kJmol-⁻¹, ΔH‡ = 41.0 ± 0.7 kJmol-⁻¹, and ΔS‡ = -20 ± 3 JK-⁻¹mol-⁻¹.