We tested two predictions required to support the hypothesis that anthropogenic acidic episodes might explain the poor biological response of upland British streams otherwise recovering from acidification: (i) that invertebrate assemblages should differ between episodic and well-buffered streams and (ii) these effects should differentiate between sites with episodes caused by anthropogenic acidification as opposed to base-cation dilution or sea-salt deposition. Chronic and episodically acidic streams were widespread, and episodes reflected acid titration more than dilution. Nonmarine sulphate (16–18% vs. 5–9%), and nitrate (4–6% vs. 1–2%) contributed more to anion loading during episodes in Wales than Scotland, and Welsh streams also had a larger proportion of total stream sulphate from nonmarine sources (64–66% vs. 35–46%). Sea-salts were rarely a major cause of episodic ANC or pH reduction during the events sampled. By contrast, streams with episodes driven by strong anthropogenic acids had lower pH (5.0±0.6) and more dissolved aluminium (288±271 μg L−1) during events than where episodes were caused by dilution (pH 5.4±0.6; 116±110 μg Al L−1) or where streams remained circumneutral (pH 6.7±1.0; 50±45 μg Al L−1). Both biological predictions were supported: invertebrate assemblages differed among sites with different episode chemistry while several acid-sensitive species were absent only where episodes reflected anthropogenic acidification. We conclude that strong acid anions – dominantly nonmarine sulphate – still cause significant episodic acidification in acid-sensitive areas of Britain and may be a sufficient explanation for slow biological recovery in many locations.