The Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) measures water vapour concentration in the stratosphere and mesosphere. The version 4 algorithm used at present to retrieve the concentration from the radiance measurements has a number of limitations. We describe work which attempts to circumvent some of these limitations. The two main limitations of the current algorithm are that (1) The retrieval assumes linearity and (2) The concentration is retrieved onto a grid with a 5.3 km spacing. The theoretical vertical resolution of the instrument is better than this. To overcome limitation (1) we use a non-linear iterative retrieval; we overcome limitation (2) by using a vertical grid with a 2.6 km spacing. We show that both these measures result in a useful improvement. These improvements are planned to be incorporated in version 5 algorithms now under development for MLS.