Upon mixing of two polyelectrolytes with opposite charges inter-polyelectrolyte complexes (IPECs) can be formed. If an additional hydrophilic, but uncharged, block is present, these complexes are likely to be stable in solution, even at an equal charge ratio.In this study a double hydrophilic blockcopolymer (polyethylene glycol-b-polyacrylic acid; PEG-b-PAA) is mixed with a bio-polyelectrolyte of opposite charge (amino modified cellulose) whereupon water soluble complexes are formed. In addition, small active molecules (drugs) can be incorporated into the core of the formed IPECs, protected by a water-soluble corona, which turns these systems into an interesting biocompatible delivery vehicle. In our study we perform a systematic investigation of the effects of various parameters (bio-polymer structure, pH, mixing ratio) onto the structure of the complexes by application of static and dynamic light scattering (SLS, DLS) and small-angle neutron scattering (SANS). Combined with i.a. thermodynamic information from ITC and fluorescence correlation microscopy (FCS) (for drug solubilization/release), this knowledge will enable the design of tailored aggregates for delivery purposes.