Mass cytometry facilitates high-dimensional, quantitative, single-cell analysis. The method for sample multiplexing in mass cytometry, called mass-tag cellular barcoding (MCB), relies on the covalent reaction of bifunctional metal chelators with intracellular proteins. Here, we describe the use of osmium and ruthenium tetroxides (OsO4 and RuO4 ) that bind covalently with fatty acids in the cellular membranes and aromatic amino acids in proteins. Both OsO4 and RuO4 rapidly reacted and allowed for MCB with live cells, crosslinked cells, and permeabilized cells. Given the covalent nature of the labeling reaction, isotope leaching was not observed. OsO4 and RuO4 were used in a 20-sample barcoding protocol together with palladium isotopes. As mass channels occupied by osmium and ruthenium are not used for antibody detection the number of masses effectively utilized in a single experiment is expanded. OsO4 and RuO4 can therefore be used as MCB reagents for a wide range of mass cytometry workflows. © 2016 International Society for Advancement of Cytometry.