A comparative study on the interaction of different PEG-containing diblock copolymers including SA400, SA600, SA1500 and OA1500 (stearyl and oleyl esters of polyethylene glycol with 400, 600 and 1500 molecular weights, respectively) with bovine serum albumin (BSA) was carried out using isothermal titration calorimetry (ITC), attenuated total reflectance Fourier transform infrared (ATR-FTIR), circular dichroism (CD), and fluorescence spectroscopies. ITC data show that SA400, SA600, SA1500 and OA1500 bind to BSA, with association constants of (14.5, 3.16, 50.7 and 17.6)x10(3) M(-1), respectively. Results also show that the binding is enthalpically driven, disfavored by conformational entropy. Quantitative analysis of the FTIR absorbance spectra at amide I' (1600-1700 cm(-1)) as well as far UV circular dichroism data show that these polymers do not disturb the BSA structure and only cause a slight increment in helicity along with a slight decrease in the beta-structure. Only stearyl esters SA400 and SA1500 slightly decreased the random structure content of the BSA. The diblock copolymers inhibit protein aggregation and bind to BSA better than their constituent PEGs causing an increment in its Tm; SA1500 is showing the strongest effect.