There has been a resurgence of interest in complex coacervation, a form of 
liquid-liquid phase separation (LLPS) in systems of oppositely charged 
macroions, but very few reports describe the somewhat anomalous coacervation 
between acidic and basic proteins, which occurs under very narrow ranges of 
conditions. We sought to identify the roles of equilibrium interprotein 
complexes during the coacervation of β-lactoglobulin dimer (BLG2) with 
lactoferrin (LF) and found that this LLPS arises specifically from LF(BLG2)2. We 
followed the progress of complexation and coacervation as a function of r, the 
LF/BLG molar ratio, using turbidity to monitor the degree of coacervation and 
proton release and dynamic light scattering (DLS) to assess the stoichiometry 
and abundance of complexes. Isothermal titration calorimetry (ITC) showed that 
initial complex formation is endothermic, but a large exotherm related to 
coacervate formation obscured other regions. On the basis of turbidimetry, 
proton release, and DLS, we propose a speciation diagram that presents the 
abundance of various complexes as a function of r. Although multiple species 
could be simultaneously present, distinct regions could be identified 
corresponding to equilibria among particular protein pairs.