Calculations are presented for the structure and the isomerization reaction of serine in aqueous solution. Polarizable continuum model (PCM) is employed to determine the effects of solvent on the stability of serine zwitterion relative to the canonical form and the proton transfer paths for the zwitterion/canonical isomerization reaction in the solution phase. The discrete/continuum model is also adopted to analyze the solute-solvent interactions. We find that the latter model gives much more accurate relative stability of canonical and zwitterion forms of serine than PCM in the aqueous solution. (C) 2005 Elsevier B.V. All rights reserved.