The measurement of the moment-curvature characteristic for thin sheet metal in the elastic-plastic range provides important information which is not readily obtained from the tensile test. Steel sheet which is roll-formed to products such as roofing typically contains a residual stress distribution and is subject to strain ageing; these conditions can be identified from bend tests. The accurate determination of the moment-curvature diagram is not easy as the limiting elastic curvature is large and the magnitude of the moment is small.Part I of this paper describes equipment and data processing which have been developed for this purpose and in Part II, results obtained in sheet which has been progressively aged and stress-relieved are presented.