In this comprehensive study, interaction of human serum albumin (HSA) with poly(acrylic acid) (PAA) was explored using Small Angle X-ray Scattering (SAXS) combined to chromatography. Results revealed the formation of a complex between HSA macromolecules and PAA chains but solely at some specific conditions of ionic strength and pH of the medium. In fact, this binding has found to take place only at pH close to 5 and low ionic strength (0.15M). Otherwise, for higher pH and salts concentration 0.75M the HSA-PAA complex tends to dissociate completely showing the reversibility of the complexation. The assessment of the influence of the HSA/PAA molar ratio on the complex radius of gyration suggests that 4 HSA molecules could bind to each PAA 100kDa chain. Besides the Porod volume evaluation for the same range of HSA/PAA ratio confirms this assumption. Finally, an all atom SAXS modelling study using BUNCH program has been conducted to find a compatible model that fit HSA-PAA complex scattering data. This model allows to portray the HSA/PAA complex as a pear-necklace assembly with 4 HSA on the 100 kDa PAA chain.