The detection of the trans (anti) conformer of nitrous acid (t-HONO) was reported in 2019 by Coutens and coworkers toward the protostellar source IRAS 16293-2422 B. The cis (syn) conformer was not detected. The detection was made through the ALMA-PILS survey, which is probably the only astronomical survey that has been used to generate its own orchestral composition (although there is another short piece based on ALMA data). The primary coverage of the survey is 329 to 363 GHz, where the rotational transitions of numerous complex organic molecules can also be found.

Nitrous acid is the smallest acid that contains nitrogen. When deprotonated, the NO–
2 nitrite anion is obtained. Nitrous acid is less stable that nitric acid, HNO₃, which has an additional oxygen atom datively bonded to the nitrogen 2s² pair.