A turbine for a turbo-machine, comprising: a turbine wheel having an axis; a turbine housing defining two volutes (119a, 119b; 219a, 219b; 319a, 319b), the volutes having respective radially inner openings separated by a radially extending divider wall (120; 220; 320; 420), the turbine wheel being positioned within the turbine housing for rotation about the axis (101; 201; 301), and the turbine housing including a diffuser space (140; 240; 340; 440) defined between two shroud surfaces (123 124; 223, 224; 323, 324; 423, 424) and providing a gas flow path between the volutes and the turbine wheel; wherein the diffuser space includes a first portion at a first radial position, the first portion of the diffuser space having a axial extent, averaged over all circumferential positions around the rotational axis, which is lower than the axial extent of a second portion of the diffuser space, averaged over all circumferential positions around the rotational axis, at a second radial position, the second portion of the diffuser space being closer to the rotational axis than the first portion of the diffuser space; characterized in that, denoting the sum of the respective axial widths of the volutes measured at a radially-inner tip (122; 222; 322; 422) of the divider wall (120; 220; 320; 420) and averaged over all circumferential positions around the rotational axis (101; 201; 301) by W inlet, and an axial spacing of the shroud surfaces (123 124; 223, 224; 323, 324; 423, 424) at the radially-outermost portion of the turbine wheel as W throat, a throat-to-inlet ratio defined as W throat W inlet is in the range 1.05 to 1.3. a turbine wheel having an axis; a turbine housing defining two volutes (119a, 119b; 219a, 219b; 319a, 319b), the volutes having respective radially inner openings separated by a radially extending divider wall (120; 220; 320; 420), the turbine wheel being positioned within the turbine housing for rotation about the axis (101; 201; 301), and the turbine housing including a diffuser space (140; 240; 340; 440) defined between two shroud surfaces (123 124; 223, 224; 323, 324; 423, 424) and providing a gas flow path between the volutes and the turbine wheel; wherein the diffuser space includes a first portion at a first radial position, the first portion of the diffuser space having a axial extent, averaged over all circumferential positions around the rotational axis, which is lower than the axial extent of a second portion of the diffuser space, averaged over all circumferential positions around the rotational axis, at a second radial position, the second portion of the diffuser space being closer to the rotational axis than the first portion of the diffuser space; characterized in that, denoting the sum of the respective axial widths of the volutes measured at a radially-inner tip (122; 222; 322; 422) of the divider wall (120; 220; 320; 420) and averaged over all circumferential positions around the rotational axis (101; 201; 301) by W inlet, and an axial spacing of the shroud surfaces (123 124; 223, 224; 323, 324; 423, 424) at the radially-outermost portion of the turbine wheel as W throat, a throat-to-inlet ratio defined as W throat W inlet is in the range 1.05 to 1.3.