[en] The first longitudinal coherence measurement of the transient inversion collisional x-ray laser is presented. The scheme under study is the picosecond output of the Ni-like Pd x-ray laser at 14.68 nm generated by the C0MET laser facility at LLNL. Interference fringes were generated using a Michelson interferometer setup in which a thin multilayer membrane was used as a beam splitter. Longitudinal coherence measurements were made for this transition by changing the length of one interferometer arm and measuring the resultant variation in fringe visibility. The nature of this dependence also allows for an estimation of the linewidth of the lasing transition to be made. Analysis indicates a linewidth of ∼0.3 pm which is a factor of four less than previous measurements on quasi-steady state x-ray laser schemes