This work deals with the influence that the addition of poly(1,4-butanediol) tolylene 2,4-diisocyanate terminated prepolymer (PBTDI) at different stages during the manufacturing process of a traditional lithium lubricating grease exerts on its rheological and morphological characteristics. With this aim. the reaction between terminal isocyanate groups and the hydroxy group located in the hydrocarbon chain of the 12-hydroxystearate lithium soap was intended to be promoted during lubricating grease processing. Several PBTDI-based greases just differing in the process stage at which this polymer was added were prepared. The polymer used and the final lubricating greases were characterized by FTIR spectroscopy and DSC techniques. The effect of PBTDI addition at different processing stages was tested on final greases by performing small-amplitude oscillatory shear (SAOS) measurements as well as atomic force microscopy (AFM) observations. Addition of PBTDI significantly increased the values of the linear viscoelasticity functions when it was added during the final cooling stage of the manufacturing process. It was demonstrated that this increment in relation to the values found for the additive-free lubricating grease was only observed when PBTDI chemically interacted with the soap network. However, the effectiveness of PBTDI as rheology modifier was much lower when the addition took place at earlier steps of the manufacturing process. Some competitive reactions due to either the presence of water or the high temperatures applied have been proposed to explain this loss of effectiveness depending on the lubricating grease processing stage at which PBTDI was added. (C) 2006 Elsevier B.V. All rights reserved.