Stable isotope studies for paleoenvironmental reconstruction are reviewed. Mass spectrometry has been an essential technique for these types of studies. The target elements, such as carbon, sulfur, and nitrogen, are converted into gaseous species, CO₂, SO₂, and N₂, respectively. The gaseous products are then introduced into the mass spectrometer and their isotopic compositions are determined. Two types of gas inlet systems (dual-inlet and continuous flow) has been proposed for isotope analyses. Continuous-flow isotope ratio mass spectrometry (CF-IRMS) is a more recent development compared to the dual inlet system. The dual inlet system requires high-purity sample gases prepared offline, whereas the CF-IRMS system introduces sample gases prepared in a continuous flow of helium to the mass spectrometer after separation by gas chromatography. The continuous flow technique has significant advantages over dual inlet in terms of a higher sensitivity and shorter analysis time. The precision of isotope analysis by CF-IRMS is believed to be lower than that of the off-line dual inlet system, but a high-precision, similar to that obtained using off-line methods, has been achieved recently, even for CF-IRMS. The present paper describes research on the Cretaceous–Paleogene (K–Pg) mass extinction event as an example of reconstructing paleoenvironments using light stable isotope compositions.