One of the smaller Coenagrions. The males are light blue with black markings and bands on the abdomen. The characters for sure identification are small and should be observed carefully. The pterostigma is diamond-shaped (lozenge) and is blackish-brown with a lighter coloured border. The black marking on S2 is characteristic and resembles the helmet of Mercury, who according to roman mythology wore a winged helmet. The shape also looks like a U placed on a spearhead. It is always connected to the hind margin of S2. The females are green-blue with black markings. A correct identification of the females is difficult and can only be done with certainty by checking the shape of the pronotum.
The main range of C. mercuriale covers France and the northern part of the Iberian Peninsula, where it is widespread and common. The species has a more scattered occurence in the southern half of spain and in Italy, where it is lacking north of the Po river. It is missing from all Mediterranean islands. In central Europe it is found in Germany and western Austria, where it is widespread but rare, with small local populations. In Belgium it is very rare, only found in two small areas in southern Wallonia. In the Netherlands it is considered extinct, although vagrants have been found. In Great Britain it is restricted to a small number of populations in Wales and the south and west of England.
Coenagrion mercuriale is found at unshaded, often calcareous runnels, small streams and irrigation ditches, preferably with a rich aquatic and riparian vegetation of Watercress and Fool's waterdress. The water is typically shallow and slow-flowing over a gravel or marl bed with patches of organic detrius. The vegetation near the banks often consists of low herbs and grasses and is frequently used for hay production or grazing. It is one of the few species that is mainly found in extensive agricultural areas. here it benefits from mowing of bank side vegetation and cleaning of waterways which prevents the habitat from becoming overgrown and shaded. In Great Britain the species is found runnels and streams in acidic heathland, chalk streams and calcareous fens. C. mercuriale is mostly found below 700 m in the middle latitudes, but can reach 1 500 m in the Iberian Peninsula (and over 2 100 m in Morocco).