We report on a high-sensitive SQUID-susceptometer for magnetization studies on two-dimensional electron systems (2DES). It consists of a low-noise thin-film dc SQUID and a wire-wound first-order gradiometer and is operated inside a superconducting 10 T-magnet. At zero magnetic field the system's flux noise level is frequency-independent and only 40×10−6Φ0/√Hz, which is almost the noise level of the bare SQUID. In magnetic fields up to 10 T the sensitivity can be better than 10−13 J/T at frequencies above 1 kHz. With this setup, we have studied the de Haas–van Alphen effect on a gated single layer 2DES revealing spin splitting and the enhanced Landé factor.