Abstract. We prove that for any free ergodic nonsingular nonamenable action Γ (X, µ) of all Γ in a large class of groups including all hyperbolic groups, the associated group measure space factor L ∞ (X) ⋊ Γ has L ∞ (X) as its unique Cartan subalgebra, up to unitary conjugacy. This generalizes the probability measure preserving case that was established in [PV12]. We also prove primeness and indecomposability results for such crossed products, for the corresponding orbit equivalence relations and for arbitrary amalgamated free products M1 * B M2 over a subalgebra B of type I.