Several methods have been investigated for the synthesis of tetraalkyl thiopyrophosphates, including one that can be carried out very rapidly with high yield. Purification of these compounds proved exceedingly difficult, because they were sensitive to traces of acids or bases, and probably to inorganic salts at temperatures above 70 °C. All methods investigated yield only the thiono isomer, there being no evidence for the corresponding P—S—P isomer. Toxicological studies do not support the results reported by Fiszer and workers. None of the compounds is more toxic intravenously than tetraethyl pyrophosphate. The synthesis of several other related compounds for infrared study is reported.