The use of linear PEGs for protein precipitation raises the issues of high viscosity and limited selectivity. This paper explores PEG branching as a way to alleviate the first problem, by using 3-arm star as the model branched structure. 3-arm star PEGs of 4,000 to 9,000 Da were synthesized and characterized. The effects of PEG branching were then elucidated by comparing the branched PEG precipitants to linear versions of equivalent molecular weights, in terms of IgG recovery from CHO cell culture supernatant, precipitation selectivity, solubility of different purified proteins, and precipitation kinetics. Two distinct effects were observed: PEG branching reduced dynamic viscosity; secondly, the branched PEGs precipitated less proteins and did so more slowly. Precipitation selectivity was largely unaffected. When the branched PEGs were used at concentrations higher than their linear counterparts to give similar precipitation yields, the dynamic viscosity of the branched PEGs were noticeably lower. Interestingly, the precipitation outcome was found to be a strong function of PEG hydrodynamic radius, regardless of PEG shape and molecular weight. These observations are consistent with steric mechanisms such as volume exclusion and attractive depletion. Biotechnol. Bioeng. 2012; 109:736746. (C) 2011 Wiley Periodicals, Inc.