Progress is reported in evaluating the mass spectra of cometary dust grains that have been obtained by impact mass analyzers flown onboard the Giotto and Vega spacecrafts. Statistical analysis of about 5000 spectra transmitted to the Earth in the more compressed modes 1-3 demonstrates that the mean elemental composition is similar to that of Cl carbonaceous chondrites with the exception of composition. The isotopic ratios do not differ from solar system values. Five classes of particles are suggested: 1) with dominant Mg-Si element group, 2) with dominant CHON abundances, 3) composites of classes 1 and 2, 4) grains rich in carbon, 5) grains rich in iron and sulphur.