A new multi-channel air ion spectrometer (AIS) is presented. The instrument allows simultaneous measurements of positive and negative ion distributions from 3.2 to 0.0013 cm 2 V-1 s(-1) (0.80-40 nm diameter or, using the Tammet correction, from 0.4 to 40 nm). The mobility range is divided into 27 fractions which are measured simultaneously to ensure a high time resolution (down to 10 seconds). The instrument calibration shows a good agreement with the mobility of electrically-classified, mono-dispersed aerosols. The spectrometer overestimates low concentrations of cluster ions. This is caused by a natural production of small ions (< I nm, 10 to 100 cm(-1)) inside the spectrometer. The instrument specifications, calibration results and measurements performed show a huge application potential of the new spectrometer.