Polymeric scale inhibitors used for downhole oilfield squeeze treatments must perform the dual role of inhibiting scale formation at low concentration levels (~2 to 20ppm) whilst giving acceptably long return curves at the wellbore. Both the inhibition efficiency and retention design aspects of polymeric scale inhibitors are thought to be functions of the molecular weight (Mw) of the polymeric species. However this has not yet been fully demonstrated experimentally.In this paper we studied three polymeric scale inhibitors viz. PPCA (phosphino polycarboxylic acid) SPPCA (Sulphonated- PPCA) and P- functionalizedcopolymer (includes phosphonate and sulphonate groups along its backbone) in some detail. This paper describes several novel contributions to the study of the effects of the polymer molecular weight distribution (MWD) on the inhibition efficiency (IE) performance of the polymeric system. Since these are polymeric SIs separate measurements of the barite IE of the precipitated polymer and its remaining supernatant solution have been compared with the “stock” polymer. Our studies also evaluated the compatibility of these inhibitors under different conditions ([Ca2+] pH and temperature) in order to understand their phase behaviour in operating conditions in the oilfield.An alternative method which we have used to study the phase separation of PPCA and SPPCA is by adding methanol (MeOH) to the polymeric solution. This process is also known as liquid-liquid extraction where the polymer is initially dissolved in one solvent and is extracted continuously from this solution by addition of another (poorer) solvent which is fully or partially miscible with the first solvent.Our findings reveal that the precipitated PPCA and SPPCA are more efficient at inhibiting barite than either the stock or supernatant. We demonstrate that this is due to the fact that the precipitate preferentially contains larger molecular weight species which have an increased IE. However the corresponding inhibition efficiency results for the P- functionalized copolymer (PFC) are rather different to those for SPPCA and PPCA and this described in the paper.The significance of these results for field precipitation squeeze treatments will be described. We believe that the ideas proposed in this work for PPCA precipitation and inhibition are generally applicable to all polymeric scale inhibitors. Thus this work will help us to develop a model for polymer precipitation (and adsorption) which can be used for designing squeeze precipitation treatments for polymeric scale inhibitors.