The functional properties of the polymeric hydrogen peroxide stabilizing agent, Poly(α-hydroxyacrylic acid) (PHA) were defined. In order to gain information on the stabilization mechanism and the optimal liquid phase properties, the stabilizing effect of PHA in liquid phase was determined over a wide range of pH and temperature. The effects of hydrogen peroxide and PHA concentrations as well as those of ferric and manganese ions were also accounted for. The experiments were conducted analyzing the escaping oxygen content by on-line gas-phase mass spectrometry. Based on the measurements a statistically significant regression model with good fit describing the system was developed and its response surfaces were generated. Also a pseudo-second order kinetic expression was established for a system including the effect of PHA. It was found that without the presence of the stabilizer, the prevailing species of the added ferric ions at pH > 7.7 seemed to slow down the decomposition of hydrogen peroxide. This is probably due to the solubility properties of iron. PHA was found to have a stabilizing effect over hydrogen peroxide in the presence of 2 mg/kg metals Fe and Mn at pH 10 but the stabilization ability is limited to temperatures below ca. 60°C. Under typical bleaching conditions for mechanical pulp, i.e., pH 9 and T=70 ºC, a slight optimum of approximately 150 mg/kg in PHA content can be seen. Introduction of PHA generally exposes the system more to the effects of pH and temperature. However, in the presence of PHA and Mn, the contribution of iron to the decomposition is very small. Considering the net charge of the prevailing iron species as well as the minor effect of temperature, an interaction mechanism based on shielding is suggested to occur between PHA and iron. The ability of PHA to stabilize also pure peroxide with no added metals also promotes the shielding mechanism. However, the enhanced sensitivity of H2O2 decomposition to pH in the presence of manganese and PHA justifies the existence of an ionic interaction mechanism, too.