AbstractTo account for the non‐equilibrium character of quark‐gluon plasma formation in relativistic heavy‐ion collisions, it is proposed to use the product of energy density ϵ and interaction time τint(rather than ϵ) as the critical quantity for plasma formation. In a geometrical model, τ is approximated by the transit time, and ϵ by the maximum energy density calculated in the center‐of‐mass frame. For central collisions, an analytical expression for ϵ·τ is given. As an example, the systems16O +16O and208Pb +208Pb are investigated as functions of c.m. energy, and their respective suitability for plasma forma