Volcanic ash is known to charge electrically, producing some of the most spectacular displays of lightning in nature. Here we investigate the electrical characteristics of ash from two different Icelandic volcanoes – Eyjafjallajökull in 2010 and Grímsvötn in 2011. Laboratory tests investigated the charge transferred to a conducting plate due to fall of volcanic ash through an insulating cylinder. Ash from the Eyjafjallajökull eruption was found to charge slightly positively, whilst Grímsvötn ash was substantially negatively charged. Measurement of the volumetric ratio of particle diameters showed the Eyjafjallajökull ash to have a bimodal distribution, and the Grímsvötn ash a monomodal distribution. Previous experiments with single-material particle systems show that smaller particles charge negatively and larger ones positively [e.g. Lacks and Levandovsky 2007]. Since charge is carried by individual particles, the charging is likely to be dominated by the number size distribution, therefore the large negative charge of the Grímsvötn ash is likely to be related to a large number of small particles within the number size distribution of the ash.