Using thermomechanical spectroscopy, the molecular-weight distribution and relaxation transitions have been investigated in commercial LDPE and grafted by itaconic acid (LDPE-g-IA). This grafting in the molten LDPE was done by reactive extrusion with varied content of reactants in the blend under alternating of a shearing rate applied onto the melt. The dependence of structural relaxation changes in LDPE is shown upon the depth of the mechanochemical transformations and the competing reactions at IA grafting, and also on the chemical crosslinking of the macromolecules. The reason for MWD bimodality for LDPE-g-IA obtained in dynamic mixing is the raised homogenization degree of the reactive blend and the higher grafted product yield compared with static mixers. The mixer type substantially affects the structure of the LDPE-g-IA amorphous phase. The data obtained reflect chemical transformations of LDPE molecules in IA＇s presence without an initiator of radical reactions (DCP). Most probable is the IA initiation of molecular crosslinking reactions. There could also occur IA thermodegradation and oligomerization. LDPE and IA or products of acid chemical transformations do not agree thermodynamically (the calculated solubility parameters are 16.1 (MJ/m(3))(0.5) for polyethylene and 26.4 (MJ/m(3))(0.5) for IA). From the above procedure it can be supposed that nongrafted IA (or its oligomers) exerts an antiplastifying effect on LDPE and LDPE-g-IA.