A homopolymer bearing L-Methionine (PMet) in its side chain was synthesized by free radical polymerization of the corresponding vinyl monomer. Viscosimetric and pHmetric measurements indicate that PMet, in water at low pH, takes a compact conformation, which is destroyed (expanded) with increasing ionization of the carboxyl groups of the side chains. The influence of temperature and of some additives (neutral salt, urea, organic solvent, SDS) on the compact conformation and the pH-induced conformational transition was investigated. It was concluded that hydrogen bonds are the main factors responsible for the existence of the compact conformation.