There is described a multi step process for preparing an aqueous dispersion of heterogeneous polymer particles by mini-emulsion polymerisation, the process comprising the steps of (I) (a) forming a mixture comprising: (i) water; (ii) at least one stabiliser (such as a surfactant and a hydrophobic co-stabiliser); (iii) at least one tackifier (such as polyterpenes, rosin resins and/or hydrocarbon resins); (iv) at least one α,β-ethylenically unsaturated monomer; (b) applying high shear to the mixture from step (a) to form an essentially stable mini-emulsion comprising an aqueous continuous phase and dispersed therein stabilised droplets of average diameter from about 10 to about 1000 nm, (c) polymerising the monomer(s) within the droplets in the presence of a free radical initiator; (d) adding further monomer to the dispersed phase to form a dispersion; then: (II) using the dispersion from step (I) as a seed to form a dispersion of heterogeneous polymer particles in a subsequent emulsion polymerisation. The resultant tackified PSAs may have a core shell structure and are stable under high shear.