Pd@SnO2 and SnO2@Pd core@shell nanocomposites are prepared via a microemulsion approach. Both nanocomposites exhibit high-surface, porous matrices of SnO2 shells (>150 m(2)g(-1)) with very small SnO2 crystallites (<10 nm) and palladium (Pd) nanoparticles (<10 nm) that are uniformly distributed in the porous SnO2 matrix. Although similar by first sight, Pd@SnO2 and SnO2@Pd are significantly different in view of their structure with Pd inside or outside the SnO2 shell and in view of their sensor performance. As SMOX-based sensors (SMOX: semiconducting metal oxide), both nanocomposites show a very good sensor performance for the detection of CO and H-2. Especially, the Pd@SnO2 core@shell nanocomposite is unique and shows a fast response time (tau(90) < 30 s) and a very good response at low temperature (<250 degrees C), especially under humid-air conditions. Extraordinarily high sensor signals are observed when exposing the Pd@SnO2 nanocomposite to CO in humid air. Under these conditions, even commercial sensors (Figaro TGS 2442, Applied Sensor MLC, E2V MICS 5521) are outperformed.