The Tate Gallery (London, U.K.) has a major collection of sculpture by Naum Gabo, a pioneer constructivist sculptor. One in particular Construction in space: 2 cones , was showing serious signs of deterioration and it was queried why. Observations, identification and testing of the artists materials revealed the use of polymethylmethacrylate, cellulose nitrate, cellulose, acetate and casein sheet. The progressive deterioration of Construction in space is described. Problem plastics were found to be cellulose acetate rather than cellulose nitrate. It is suggested that until more is known about degradation mechanisms, stable environments are essential for other such objects. 30 references.