The Versailles Project on Advanced Materials and Standards 'VAMAS' initiated in 1996 a Technical Working Area 'TWA20' entitled 'Measurement of residual stress'. The project was designed to facilitate the basic research necessary for, and to draft, the first international standard for the measurement of residual stresses using neutrons. An ISO/VAMAS TTA was delivered in 2001 as a precursor to a full standard. A substantial part of the project was a series of four `Round Robin' inter-laboratory comparative measurements on single and multiphase materials containing residual stresses which were introduced by a variety of fabrication techniques and in-service loads. One of the 'Round Robin' studies involved the measurement of the residual stress field in a ferritic steel weld. This paper describes the investigation and outlines the results of the measurements and its implications for the neutron diffraction standard.