On 17 May 2010, the FAAM BAe-146 aircraft made remote and in situ measurements of the volcanic ash cloud from Eyjafjallajökull over the southern North Sea. The Falcon 20E aircraft operated by Deutsches Zentrum für Luft- und Raumfahrt (DLR) also sampled the ash cloud on the same day. While no “wingtip-to-wingtip” co-ordination was performed, the proximity of the two aircraft allows worthwhile comparisons. Despite the high degree of inhomogeneity (e.g., column ash loadings varied by a factor of three over ∼100 km) the range of ash mass concentrations and the ratios between volcanic ash mass and concentrations of SO2, O3 and CO were consistent between the two aircraft and within expected instrumental uncertainties. The data show strong correlations between ash mass, SO2concentration and aerosol scattering with the FAAM BAe-146 data providing a specific extinction coefficient of 0.6–0.8 m2 g−1. There were significant differences in the observed ash size distribution with FAAM BAe-146 data showing a peak in the mass at ∼3.5μm (volume-equivalent diameter) and DLR data peaking at ∼10μm. Differences could not be accounted for by refractive index and shape assumptions alone. The aircraft in situ and lidar data suggest peak ash concentrations of 500–800 μg m−3with a factor of two uncertainty. Comparing the location of ash observations with the ash dispersion model output highlights differences that demonstrate the difficulties in forecasting such events and the essential nature of validating models using high quality observational data from platforms such as the FAAM BAe-146 and the DLR Falcon.