The thermal stability of Langmuir-Blodgett-Kuhn (LBK) multilayers of poly-[(gamma-methyl L-glutamate)-co-(gamma-n-octadecyl L-glutamate)] (PG) has been investigated using neutron reflectometry. The stability of the LBK films of such "hairy rod" molecules was determined by measuring the interfacial width between protonated and deuterated stacks of PG. No center of mass diffusion of molecules between layers of protonated and deuterated material between 349 and 414 K was found. The broadening of the interface is lower than 10(-18) cm(2)/s in this temperature region. The film structure and its changes during annealing of these rodlike molecules with flexible side chains were also probed with X-ray reflectometry and compared to the results of the neutron experiments. The total film thickness decreased during annealing while no loss of material due to thermal desorption occurred. The origin of the thermal stability against interdiffusion is discussed.