The complexation between hen egg white lysozyme (HEWL) and a novel pH-sensitive and intrinsically hydrophobic polyelectrolyte poly(sodium(sulfamate-carboxylate)isoprene) (SCPI), was investigated by means of dynamic, static, and electrophoretic light scattering and isothermal titration calorimetry measurements. The complexation process was studied at both pH 7 and 3 (high and low charge density of the SCPI, respectively) and under low ionic strength conditions for two polyelectrolyte samples of different molecular weights. The solution behavior, structure, and effective charge of the formed complexes proved to be dependent on the pH, the [-]/[+] charge ratio, and the molecular weight of the polyelectrolyte. Increasing the ionic strength of the solution led to vast aggregation and eventually precipitation of the complexes. The interaction between HEWL and SCPI was found to be mainly electrostatic, associated with an exothermic enthalpy change. The structural investigation of the complexed protein by fluorescence, infrared, circular dichroism spectroscopic, and differential scanning calorimetric measurements revealed no signs of denaturation upon complexation.