A possible accident scenario in a gas-cooled fast reactor (GCFR) is the leakage of secondary steam into the core. A full-scale experimental study of the physics effects of such an accidental condition has been performed on the zero power reactor (ZPR)-9 critical facility at Argonne National Laboratory. Polyethylene foam strips were used to simulate steam for these measurements. The basic neutronics parameters, namely, neutron spectrum, spectral indexes, reactivity worths, 238U Doppler effect, and B4C control rod worths, were measured in the steam-flooded GCFR critical assembly and also in the corresponding dry, reference GCFR assembly. The results of these measurements clearly show the spectrum softening effects on steam entry. For the analysis of the experiments, ENDF/B-IV-based data were used with two-dimensional diffusion theory methods. It was concluded that the values of the primary safety parameters increased upon steam entry relative to the reference dry case. Such an increase would mitigate the effects of accidental steam entry in a GCFR.