Thionitrous acid (HSNO), a potential key intermediate in biological signaling pathways, has been proposed to link NO and H2S biochemistries, but its existence and stability in vivo remain controversial. We establish that HSNO is spontaneously formed in high concentration when NO and H2S gases are mixed at room temperature in the presence of metallic surfaces. Our measurements reveal that HSNO is formed by the reaction H2S + N2O3 → HSNO + HNO2, where N2O3 is a product of NO disproportionation. These studies also suggest that further reaction of HSNO with H2S may form HNO and HSSH. The length of the S-N bond has been derived to high precision and is found to be unusually long: 1.84 Å, the longest S-N bond reported to date for an R-SNO compound. The present structural and, particularly, reactivity investigations of this elusive molecule provide a firm foundation to better understand its potential physiological chemistry and propensity to undergo S-N bond cleavage in vivo.