The optical data storage medium (20) has a substrate (1) with a surface area having tracks (4) of marks representing stored information. The information is readable by means of a scanning radiation beam (10). A reflective layer (2) is present, reflecting the radiation beam (10). The medium has a radiation beam (10) reflection larger than 50%. The reflective layer (2) comprises a compound of the formula Al.sub.100-y,X.sub.y, in which formula Al is aluminum and X is an element selected from the group of Ge and Si. The letter y is the atomic fraction of X in at. %. The value of y is in the range 5