Different N-substituted derivatives of urea, thiourea and guanidine have been synthesised and characterised for structural information by FT-IR and C-13 NMR spectroscopy. Accelerated stability tests have been done to evaluate their sediment inhibition and dispersant characteristics on distillate fuels containing high proportions of unstable components and results discussed. It is inferred that urea and guanidine derivatives not only inhibit the reactions responsible for sedimentation in the test fuel but also effectively disperse the gum and sediment formed. It is concluded that the nature of substituents on the central carbon atom and on the amino groups determines the performance behaviour of these compounds. (C) 2002 Elsevier Science Ltd. All rights reserved.