The UARS Microwave Limb Sounder (MLS) measured water vapor in the middle atmosphere between September 1991 and April 1993. The current version of the data, version 4, and its predecessor, version 3, have been used in many scientific studies. As part of the process of developing the next version of UARS MLS data, a new prototype retrieval for the stratosphere/mesosphere water vapor product was developed at the University of Edinburgh. The main improvements made were (1) corrections for systematic errors and (2) doubling of the vertical resolution of the retrieval grid. Initial results were sufficiently encouraging that the code was used to produce data for all UARS days on which the MLS 183 GHz radiometer was operational. This paper describes these data and examines their quality. Comparisons are made with the available correlative data and it emerges that the prototype agrees better with the correlative data than does version 4. Agreement with frost point hygrometers is excellent in the lower stratosphere and agreement with a ground-based microwave is satisfactory in the upper stratosphere and mesosphere. Agreement with two solar occultation instruments, Atmospheric Trace Molecular Spectroscopy Experiment (ATMOS) and Halogen Occultation Experiment (HALOE), is better than it was for earlier MLS versions but the prototype data are uniformly drier than both of these instruments. We compare the data with version 4, showing several features which are more clearly visible in the new prototype. These include seasonal cycles in the upper mesosphere and equatorial lower stratosphere. ¿ 1999 American Geophysical Union