this is an original study on the effect of a polyisobutylene – polydimethylsiloxane (pibpdms) block copolymer modifier used as an interfacial active agent on the dynamics behaviour of single newtonian drops suspended in a polyisobutylene (pib) newtonian matrix. the results were divided in two sections. the first part included the experiments carried out on the non-modified and 2% block copolymer modified polydimethylsiloxane (pdms) drops of five different mw suspended in pib920 matrix. interfacial tension measurements carried out on both clean and block copolymer modified drops indicated lower values for compatibilized drops compared with the clean ones. the presence of block copolymer changed the deformation and breakup modes of compatibilized drops and decreased the flow strength required to both deform and breakup them compared to corresponding clean counterparts. however, the extent of deformation at both steady and transient states increased. the clean drops followed the classical breakup mode including deformation into a symmetrical ellipsoid followed by a cylindrical shape and finally breakup into two major droplets and some minor ones. in the second part, the effect of various concentrations of the block copolymer (0.5, 2 and 10 wt%, drop based) was studied on drop dynamics. pdms drops of two viscosity ratios, p = 0.036 and p = 1.13, were selected. for comparison purposes, the same experiments were also carried out on the corresponding clean drop counterparts. the compatibilized drops of both viscosity ratios presented a higher deformation with respect to the corresponding clean ones. the compatibilization concentrations lay below, equal to and beyond the critical concentration of block copolymer on the drop surface, 0.5% < cs = 2% <10%. the results for initial stages of deformation of pdms drops of both viscosity ratios were explained by saturation concentration of block copolymer on the drop surface, s c . at higher shear rates, the deformation of low viscosity ratio copolymer modified drops was governed by drop tipstretching, non-uniformity of the block copolymer on the drop surface and dilution effect as well. in case of higher viscosity ratio drops, parameters such as drop viscosity, lack of internal circulation and difficulty of the blocks mobility in the corresponding similar phases were determining factors for drop deformation and breakup.