The wingspan is 10–13 mm. Adults are on wing throughout the year, but are most common in July and August in Great Britain. The larvae have been recorded feeding on rosebay willowherb (Chamerion angustifolium), great willowherb (Epilobium hirsutum), broad-leaved willowherb (Epilobium montanum), marsh willowherb (Epilobium palustre) and evening primrose (Oenothera species), but great willowherb is the main food plant. The other listed plants might be misidentifications. Young larvae probably live as leaf miners. Older larvae live, mostly communally, in spun uppermost leaves.

The wingspan is 7–11 millimetres (0.3–0.4 in). Adults are on wing in May and again in August. There are two generations per year. The moth is small and distinctively marked. The species could be confused with Mompha locupletella, but it lacks that species contrasting dark and light patches at the base of the forewing.

Triclonella determinatella is a moth in the family Cosmopterigidae. It is found in North America, where it has been recorded from Alabama, Arkansas, Florida, Illinois, Kansas, Louisiana, Mississippi, Oklahoma and Texas. The wingspan is about 10 mm. Adults have been recorded on wing in January and from March to October.