The aim of this study was to determine particle size distributions (PSDs) generated by six pocket inhalers (2 × MDIs – metered dose inhalers and 4 × DPIs – dry powder inhalers) used in the treatment of asthma and chronic obstructive pulmonary disease (COPD). Two methods were compared – cascade impactor and time-of-flight online spectrometer. The results showed that the main modes positions of mass PSDs obtained by either of the methods were comparable. During the measurements of number PSDs by using of APS spectrometer, we measured bimod-al distributions for MDI inhalers and monomodal ones for DPI inhalers. The differences of PSDs measured under ambient and respiratory tract conditions were minimal and did not influence the deposition area of inhalants. The results showed that the determination of the PSDs by online spectrometer is a suitable alternative to the traditional methods of measuring the particle size distributions of inhalation aerosol.