By using bulk samples, rock magnetic measurements were performed to discriminate between pyrrhotite- and greigite-bearing shallow marine sediments that are now uplifted above sea level in southwestern Taiwan. Thermal demagnetization of a composite isothermal remanent magnetization (IRM) was found to be effective in differentiating between the two types of sediments. To check the thermal instability and estimate the true unblocking temperature (TB) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature step during demagnetization. While pyrrhotite-bearing samples showed unambiguous TB temperature spectra, greigite-bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instability of greigite is a practical and important clue for its identification. Zero-field warming of IRM from 5 to 300 K sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation. ¿ American Geophysical Union 1996