2D heterostructures are a highly versatile platform for obtaining materials with tailored properties. However, to achieve heterostructures with intentionally engineered phases, developing synthetic approaches capable of precise phase control is important. Hua Zhang and colleagues report the synthesis of transition metal dichalcogenide (TMD) heterostructures composed of layers of MoS2 in different phases simply by changing the growth atmosphere. As a first step, single-layer 2H-MoS2 nanosheets — in which the MoS2 is in a trigonal prismatic phase — are grown on a mica substrate using pure argon as the carrier gas. Subsequently, 1 T′-MoS2 nanoribbons — where the Mo atoms are coordinated in an octahedral arrangement — are synthesized on top of the 2H-MoS2 layer by altering the carrier gas composition from pure argon to a mixture of argon and H2. This change in gas composition facilitates the creation of heterophase structures and provides a blueprint for the phase engineering of TMD heterostructures.