Synchrotron-based x-ray topography (XRT) measurements have been used to study the initial stages of relaxation in ZnSe layers grown by molecular beam epitaxy on vertical gradient freeze Bridgman GaAs substrates. The formation of the very first strain-relieving misfit dislocations in the grown ZnSe layers has been detected in a layer of thickness 100 nm. No such dislocations have been observed in a corresponding layer of 95 nm thickness. The critical thickness for this material system is therefore estimated to be 97.5±2.5 nm, which is markedly lower than the widely accepted value of 150 nm. In contrast to the InGaAs/GaAs system, combined XRT and transmission electron microscopy studies indicate that the initial misfit dislocations observed for ZnSe/GaAs are not, in general, formed by the bending over of pre-existing threading dislocations into the interface, but by other mechanisms such as stacking fault decomposition. The critical thickness data obtained have been used to infer the maximum critical thickness of CdZnSe quantum wells possible in II-VI laser diodes. © 1998 American Institute of Physics.