A double‐oven cracking furnace has been used to produce a flux composed of smaller selenium molecular species for the molecular‐beam epitaxy (MBE) growth of ZnSe on (100) GaAs substrates. Mass spectrometric analysis of fluxes from a conventional effusion cell and the cracking oven showed a significant difference in the number of large selenium molecules Sen(n>2) in their mass spectra. This new selenium source was found to be reactive with GaAs and its oxides, therefore its reaction with the substrate reduced the effectiveness in desorbing the oxide layer by heat treatment. Using the cracked selenium source, unintentionally doped ZnSe films have been grown on GaAs substrates desorbed in a separate ultrahigh vacuum (UHV) chamber linked to the MBE system. These undoped layers were highly resistive at room temperature and had low‐temperature photoluminescence spectra dominated by emission from the recombination of free excitons, comparable to the best layers grown using an uncracked selenium source.