The objective of this study is to examine the phase modification of styrene ethylene butylene-styrene (SEES) block copolymer by different additives and its influence on morphology and mechanical, and dynamic mechanical properties. The additives chosen are the coumarone-indene (CI), phenol-formaldehyde (PF), paraffin hydrocarbon (PAHY) resins, as well as aromatic oil (AO), polystyrene (PS), polypropylene (PP), ethylene vinyl acetate (EVA) (VA 28 and 45%), and ethylene propylene diene monomer (EPDM) rubber. It is interesting to note that of all the additives, PP has the most prominent effect. The mechanical properties of SEES polymer are enhanced to a large extent by PP. The value of tan delta maximum of SEES at both the low and the high temperature transitions is decreased. All the resins and PS increase the storage modulus and the tensile modulus of the SEES polymer. CI resin and AO modify the hard and soft phases of SEES polymer. AO, EPDM rubber, and EVA lower the mechanical strength of the SEES polymer. The results are explained on the basis of morphology studied with the help of scanning electron microscopy.