The signal-triggered amplification system (SAS) consists of two reactions in aqueous buffer (Figure ). First, we activate a phosphine (PR3) compound by allylic substitution on a quaternary allylphosphonium ion (PR4 + ) with thiol nucleophiles (SH-signal), which forms an allylic reaction product (Nuc. Product). In the second reaction, the liberated trivalent phosphine reduces a disulfide bond resulting in the formation of two thiol equivalents and the production of phosphine oxide (O=PR3) (Figure ). The two new thiol molecules can perform another allylic substitution reaction on remaining allylphosphonium salt (P-salt), leading to liberation of more phosphine and subsequent disulfide reduction, propagating the cycle. As this iterative process continues, the quantity of thiol molecules is amplified, until all neutral phosphine is consumed.