Methacrolein trans-2-Methyl-2-butenal Citronellal or rhodinal (C10H18O) is a monoterpenoid aldehyde, the main component in the mixture of terpenoid chemical compounds that give citronella oil its distinctive lemon scent.

Citronellal is a main isolate in distilled oils from the plants Cymbopogon (excepting C. citratus, culinary lemongrass),[3] lemon-scented gum, and lemon-scented teatree.

The (S)-(−)-enantiomer of citronellal makes up to 80% of the oil from kaffir lime leaves and is the compound responsible for its characteristic aroma.

Citronellal has insect repellent properties, and research shows high repellent effectiveness against mosquitoes.

[4] Another research shows that citronellal has strong antifungal qualities.