Acrylamide hydrogels were synthesized in the presence of various non-solvents for linear polyacrylamide to examine phase separation during polymerization. The process was found to be dependent upon the segmental volume, the chemical structure, and the concentration of the non-solvent. The concept of conversion-phase diagram for linear polymer is introduced and used qualitatively to understand polymerization induced phase separation (PIPS), and to predict the onset of PIPS during hydrogel synthesis. (C) 2003 Elsevier Ltd. All rights reserved.