Aims:We present an analysis of high resolution spectra in the J band of five ultra cool dwarfs from M6 to L0. Methods: We make use of a new ab initio water vapour line list and existing line lists of FeH and CrH for modelling the observations. Results: We find a good fit for the Mn I 12 899.76 Å line. This feature is one of the few for which we have a reliable oscillator strength. Other atomic features are present but most of the observed features are FeH and H2O lines. While we are uncertain about the quality of many of the atomic line parameters, the FeH and CrH line lists predict a number of features which are not apparent in our observed spectra. We infer that the main limiting factor in our spectral analysis is the FeH and CrH molecular spectra.