Abstract This work studies the effect of the helix–coil transition in gelatin on the structure development in the complex forming water–gelatin–BSA system using dynamic light scattering, environment scanning electron microscopy, rheometry, differential scanning microcalorimetry, circular dichroism, fluorescence, and absorption measurements. It was established that the structure of the complexes formed and the mechanism of intermacromolecular interaction are different in the case of two conformation states of gelatin. Above the temperature of the conformational transition (40°C) intermacromolecular interaction leads to collapse gelatin macromolecules and formation compact (30nm in radius) BSA–gelatin complexes (∼6:1, mole/mole), partial stabilization of the secondary structure (increase the mean helix content), and stabilization of BSA molecules against thermo aggregation. At the same time it does not leads to an appreciable change in the thermodynamic parameters of the thermal transitions for BSA and gelatin. Below the temperature of the conformation transition (at 18°C) the interaction results in formation of the large (600–1000nm in radius) complex particles due to trapping BSA molecules into the meshes of the gelatin network and, as consequence, a substantial increase in the storage and loss moduli of the system.