An adjustable formwork for forming the corner surfaces of concrete structural members includes a first and a second formwork unit. Each formwork unit extends transversely of the other and has a surface which forms one of the concrete surfaces of the concrete structural members and these surfaces define a corner junction line about which the formwork units can be pivotally adjusted relative to one another. One of the formwork units is provided with a circular-arc shaped guide slot and the other has a pair of spaced guide pins secured to it with the pins being movably guided in the slot. The center point of the arc of the slot is located on the corner junction line. The interengagement of the pins with the slot affords a link type guidance between the formwork units. As a result, forces are transmitted at the corner junction line only in the assembly state so that the line lies practically in the plane of the formwork surface which define the concrete surfaces. The forces which occur during loading of the formwork units are transmitted in the region of the junction line between the formwork supports whereby the forces are carried away along the shortest path.