Volcanic ash plumes, emitted by the Eyjafjallajökull volcano (Iceland) in spring 2010, were observed by the lidar systems MULIS and POLIS in Maisach (near Munich, Germany), and by a CIMEL Sun photometer and a JenOptik ceilometer in Munich. We retrieve mass concentrations of volcanic ash from the lidar measurements; spectral optical properties, i.e. extinction coefficients, backscatter coefficients, and linear depolarization ratios, are used as input for an inversion. The inversion algorithm searches for model aerosol ensembles with optical properties that agree with the measured values within their uncertainty ranges. The non-sphericity of ash particles is considered by assuming spheroids. Optical particle properties are calculated using the T-matrix method supplemented by the geometric optics approach. The lidar inversion is applied to observations of the pure volcanic ash plume in the morning of 17 April 2010. We find 1.45 g m⁻² for the ratio between the mass concentration and the extinction coefficient at λ = 532 nm, assuming an ash density of 2.6 g cm^−3. The uncertainty range for this ratio is from 0.9 g m⁻² to 2.3 g m⁻². At the peak of the ash concentration over Maisach the extinction coefficient at λ = 532 nm was 0.75 km⁻¹ (1-h-average), which corresponds to a maximum mass concentration of 1.1 mg m⁻³ (0.65 to 1.7 mg m⁻³). We compare the lidar inversion results to results from an independent approach using sky radiance measurements of the CIMEL in the aureole of the Sun. We find good agreement.