Oppositely charged macromolecules interact to form two distinct phenomena: liquid-liquid phase separation (LLPS) resulting in complex coacervates, or liquid-solid phase separation (LSPS), leading to aggregates. This study deals with the complex formation of lysozyme (LYS), a basic protein, with alginate (ALG), a negatively charged polysaccharide, at pH 7, exploring various mixing ratios, total concentrations, and ionic strengths. 1 Employing a droplet-based millifluidic setup coupled with optical microscopy, we conducted a comprehensive qualitative analysis under diverse experimental conditions. 2 Our findings unveil an ionic strength-dependent pattern of aggregation and coacervation within the LYS/ALG system. Using the grey level analysis, we quantitatively and qualitatively characterized the phase separation phenomena in the system. By constructing a three-dimensional phase diagram, integrating salt, LYS, and ALG concentrations, we provide a depiction of monophasic regions, liquid-solid and liquid-liquid phase separation domains, and coexisting regions of both LSPS and LLPS. Thermodynamic characterization using isothermal titration calorimetry (ITC) highlighted distinct profiles for coacervation and aggregation, with the interaction affinity (Ka) for aggregation notably higher. Structural characterization at the nanoscale was achieved through small-angle X-ray scattering (SAXS) experiments. This study underscores the need for further exploration of additional polyelectrolyte pairs to deepen our understanding of LSPS and LLPS mechanisms, fostering a comprehensive understanding of the phase separation phenomena. Figure 1: Droplet image, optical microscopy image, and ITC thermogram obtained for aggregation (A) and coacervation (B).