We report the effect of neutral macromolecular crowders poly(ethylene glycol) 
(PEG) (8 kDa) and Ficoll (70 kDa) on liquid-liquid phase separation in a 
polyuridylic acid (polyU)/spermine complex coacervate system. The addition of 
PEG decreased both the amount of spermine required for phase separation and the 
coacervation temperature (TC). We interpret these effects on phase behavior as 
arising due to excluded volume and preferential interactions on both the 
secondary structure/condensation of spermine-associated polyU molecules and on 
the association of soluble polyU/spermine polyelectrolyte complexes to form 
coacervate droplets. Examination of coacervates formed in the presence of 
fluorescently-labeled PEG or Ficoll crowders indicated that Ficoll is 
accumulated while PEG is excluded from the coacervate phase, which provides 
further insight into the differences in phase behavior. Crowding agents impact 
distribution of a biomolecular solute: partitioning of a fluorescently-labeled 
U15 RNA oligomer into the polyU/spermine coacervates was increased approximately 
two-fold by 20 wt% Ficoll 70 kDa and by more than two orders of magnitude by 20 
wt% PEG 8 kDa. The volume of the coacervate phase decreased in the presence of 
crowder relative to a dilute buffer solution. These findings indicate that 
potential impacts of macromolecular crowding on phase behavior and solute 
partitioning should be considered in model systems for intracellular 
membraneless organelles.