A dual stack optical data storage medium (30) for recording and reading by means of a focused radiation beam (29) is described. The beam enters the medium (30) through a first radiation beam entrance face (21). The medium has at least a first substrate (1a) with on at least one side of the first substrate (1a) a first layer stack (2), comprising a first information layer (3), a second layer stack (5), comprising a second information layer (6) and a first transparent spacer layer (4) between the first layer stack (2) and the second layer stack The first layer stack (2) is present at a position closer to the first radiation beam entrance face (21) than the second layer stack (5). The first information layer (3) is one selected from the group of types consisting of a read only layer and a write once layer, and 10 the second information layer (6) is one selected from the group of types consisting of a read only layer, a write once layer and a rewritable layer. The type of the first information layer (3) is different from the type of the second information layer (6). Compatibility of the medium with read only standards is achieved while the advantages of writability and erasability are maintained. Double sided versions of the medium are also described having a 15 third and fourth layer stack (7, 10).