  We report first thermodynamic measurements of the temperature derivative of
chemical potential (d{\mu}/dT) in two-dimensional (2D) electron systems. In
order to test the technique we have chosen Schottky gated GaAs/AlGaAs
heterojunctions and detected experimentally in this 2D system quantum
magnetooscillations of d{\mu}/dT. We also present a Lifshits-Kosevitch type
theory for the d{\mu}/dT magnetooscillations in 2D systems and compare the
theory with experimental data. The magnetic field dependence of the d{\mu}/dT
value appears to be sensitive to the density of states shape of Landau levels.
The data in low magnetic field domain demonstrate brilliant agreement with
theory for non-interacting Fermi gas with Lorentzian Landau level shape.
