An irreversible optical medium for storing information by local modification of the optical properties of a recording layer under the action of a light beam with an irradiance of from 0.05 to 10 nJ .mu.m.sup.2, essentially consisting of a substrate and, applied on this, a particulate recording layer from 0.01 to 10 .mu.m thick, wherein the recording layer consists of a three-dimensional arrangement of particles having a mean particle diameter of from 4 to 40 nm, and a process for its production.