Principle: Osmium tetroxide (OsO₄) is a strong oxidizing agent that can react with unsaturated fatty acids, sulfhydryl groups (-SH) and amino groups (-NH₂) of proteins in cell membranes to form stable cross-linked complexes. At the same time, OsO₄ has the property of high electron density, which can provide good contrast under the electron microscope and is especially suitable for observing the fine structure of cell membranes and lipid droplets.

Main components: Liquid A: osmium tetroxide; Liquid B: phosphates