The σ Orionis cluster (˜3 Myr, 350 pc) is an ideal site to investigate the early evolution of substellar (brown dwarf and planetary mass) objects. To date, the cluster photometric and spectroscopic sequence of free-floaters is known for a wide mass range from 1M&sun; down to roughly 3MJup. The substellar domain covers spectral types that go from mid-M classes to the recently defined ``methane'' T-types, i.e., surface temperatures between ˜3000 K and 800 K. We derive a rising initial substellar mass function in the mass interval of 150--5MJup (dN/dM ˜ M-α, with α = 0.9 ± 0.4). We also find evidence for a extension of this mass function toward lower masses down to 2--3MJup. This indicates that the population of isolated planetary mass objects with masses below the deuterium burning threshold is rather abundant in the cluster.