This paper describes an x‐ray preionized avalanche discharge‐pumped XeCl laser capable of producing output pulses of greater than 250‐ns FWHM duration and energies of ∼1 J. This represents the longest pulse duration yet reported from such a high‐power laser. Nevertheless, the laser output pulse is shown to terminate while the laser is still apparently under strong electrical excitation. The severity of this premature laser termination has been found to be dependent on both the partial pressure of the halogen donor molecule, HCl, in the laser gas mixture, and on the magnitude of the electrical excitation rate. Investigations of the discharge homogeneity, using time‐integrated and time‐resolved photographic observation of the discharge, scattering of a short duration dye laser probe beam, and time‐integrated and time‐resolved fluorescence spectra of species within the laser gas mixture have led to the conclusion that it is the formation of small, discrete current channels in the body of the discharge which is the prime cause of the premature termination. These scatter the laser radiation and also lead to nonuniform and inefficient formation of the excimers.